1
00:00:00,770 --> 00:00:02,620
Through the.

2
00:00:13,890 --> 00:00:25,950
So I will continue with discussing what we actually learn from observing gravitational waves and what type of questions this this poses at present.

3
00:00:25,960 --> 00:00:31,860
So this is an extremely exciting time because finally it's possible to detect gravitational waves.

4
00:00:31,860 --> 00:00:37,260
And we can see in places in the universe which were originally completely hidden.

5
00:00:37,270 --> 00:00:43,740
So we can now basically see through that scene the darkness and and see what's out there.

6
00:00:44,130 --> 00:00:49,560
So this is very exciting because it's a completely new window on the universe.

7
00:00:49,560 --> 00:00:57,180
And we can look at previous instances when this happens when different electromagnetic bands were opened up for the first time.

8
00:00:57,780 --> 00:01:01,950
There was there were always very interesting new discoveries that happened.

9
00:01:02,370 --> 00:01:09,329
So when the first time radio observations were made and after that actually supermassive

10
00:01:09,330 --> 00:01:14,490
black holes were discovered because what was seen was at the centres of galaxies,

11
00:01:15,150 --> 00:01:19,920
there were very bright radio sources and these are now known to be supermassive black holes.

12
00:01:19,920 --> 00:01:28,350
As gas flows into near supermassive black holes, they shine and they can they can even outshine the whole host galaxy.

13
00:01:29,190 --> 00:01:31,739
So we found something completely new and unexpected.

14
00:01:31,740 --> 00:01:38,370
And similarly, for example, in the cosmic microwave background, of course, we see the light of the Big Bang, basically.

15
00:01:38,730 --> 00:01:42,000
And so this was a great unexpected discovery and so on.

16
00:01:42,480 --> 00:01:46,980
And so now we have the gravitational wave sky we can finally detect based on waves.

17
00:01:47,490 --> 00:01:54,300
And since five years ago or than 2000 feet, so maybe now, eight years ago.

18
00:01:55,050 --> 00:01:58,710
And so it's very exciting to see what is there to see.

19
00:01:59,340 --> 00:02:06,959
And we don't know a priori. There are predictions that there should be black holes and maybe black holes are emerging and

20
00:02:06,960 --> 00:02:11,760
neutron stars that we can finally go out and contrast the theories with the observations.

21
00:02:12,750 --> 00:02:17,550
And the Nobel Prize was already awarded for the detection of gravitational waves.

22
00:02:18,070 --> 00:02:28,440
But now this is a completely open field, because the explanation for why you see gravitational waves is actually completely open.

23
00:02:29,800 --> 00:02:38,110
And there are there's a large range of frequencies where you can you can detect gravitational waves from nano hertz,

24
00:02:38,290 --> 00:02:46,630
which Steve was just described previously up to the Hertz kilohertz regime.

25
00:02:47,050 --> 00:02:53,380
So this is legal, the Earth based instrument, and this is using pulsars as the clocks.

26
00:02:54,160 --> 00:02:57,880
It's basically radio telescopes or antennas.

27
00:02:58,300 --> 00:03:06,760
And in between, there's going to be in the future, Lisa, which is going to be a space mission that three satellites are orbiting the sun,

28
00:03:06,940 --> 00:03:15,700
trailing the earth, and they exchange laser lights and they can measure distances in space on a different scale, a much larger scale.

29
00:03:15,820 --> 00:03:20,110
And so you can detect much longer gravitational wavelengths in this way.

30
00:03:20,950 --> 00:03:27,920
And so here's the landscape of of different frequencies that these different instruments can detect.

31
00:03:28,390 --> 00:03:33,790
And this is the characteristic strain. So this is the gravitational wave amplitude that the instruments detect.

32
00:03:34,360 --> 00:03:40,270
And you can see the sensitivity of these instruments as standard procedures and different types of instruments.

33
00:03:40,600 --> 00:03:50,739
And these are the source populations that are expected. So here at very low frequencies, we expect to see supermassive black hole binary in spirals.

34
00:03:50,740 --> 00:03:59,350
So you have two supermassive black holes and on relatively large orbits of of order year periods or ten year periods.

35
00:04:00,040 --> 00:04:05,979
So these supermassive black holes are orbiting and emitting gravitational waves and disturbing the lights of pulsars.

36
00:04:05,980 --> 00:04:13,120
And these this is what we can see. And so we learn something about the abundance of supermassive black hole binaries in the universe.

37
00:04:13,960 --> 00:04:26,110
Here you can detect shorter wavelengths of descent roughly the size of this instrument, or that the wavelengths between the sun and the earth.

38
00:04:26,620 --> 00:04:31,650
And on these skills, you can detect the mergers of supermassive black holes collide.

39
00:04:32,470 --> 00:04:37,930
And so this would be hopefully in 20, 32 or maybe sometime around that period.

40
00:04:38,470 --> 00:04:48,400
And Lego is already operating and there's the European counterpart, Virgo, and also to the Japanese instrument, very similar to Lego.

41
00:04:48,940 --> 00:04:53,259
And these are detecting waves between ten hertz and 1030.

42
00:04:53,260 --> 00:04:55,120
So that's it's already operating.

43
00:04:55,360 --> 00:05:02,350
And this can detect very short wavelengths, the gravitational waves, which are produced by stellar mass black locations or neutron stars.

44
00:05:03,820 --> 00:05:10,520
Or maybe distorted balls are even. So this is what you should see.

45
00:05:10,520 --> 00:05:17,030
So we can finally detect signals which are coming from places which are completely hidden in the universe.

46
00:05:17,030 --> 00:05:24,259
So we have two black holes orbiting and they are distorting the background light of, of, of the of the universe.

47
00:05:24,260 --> 00:05:26,870
So this is how it would look like in electromagnetic bands.

48
00:05:26,870 --> 00:05:32,180
But now, of course, we can also detect directly the signal which is generated by the black list, which are gravitational waves.

49
00:05:32,720 --> 00:05:35,060
And so as the black holes come closer and closer,

50
00:05:35,060 --> 00:05:44,300
they are orbiting faster and faster and each other and then finally coalesce and form a stationary rotating black hole.

51
00:05:46,460 --> 00:05:50,090
And so this is how the wave form looks like. Initially,

52
00:05:50,120 --> 00:05:58,009
it's the two blackboards just orbiting around each other and wide orbits and but gravitational waves are carrying

53
00:05:58,010 --> 00:06:04,340
the energy from the system so that the orbits shrink and eventually the black holes coalesce in this part.

54
00:06:04,700 --> 00:06:10,940
And then a perturbed black hole settles, then the stationary black hole in this ring down phase.

55
00:06:12,840 --> 00:06:17,380
And this is what we can measure. So we can, Steve, as Steve explained very nicely.

56
00:06:18,370 --> 00:06:21,940
And so there are many signals that were already detected.

57
00:06:22,180 --> 00:06:27,790
And here are some examples. They are given names based on the dates that they were detected on.

58
00:06:28,540 --> 00:06:31,910
And there's now a whole array of these detections.

59
00:06:32,560 --> 00:06:37,960
And just by measuring the signals of the gravitational waves, you can learn about the sources that generate these signals.

60
00:06:38,410 --> 00:06:44,980
And, for example, you can then you can just measure or infer the masses of these merging objects.

61
00:06:45,400 --> 00:06:51,210
And here are the different sources. In no particular order that were observed in gravitational waves.

62
00:06:51,220 --> 00:07:00,520
You can see that some of these have masses up to 20 or 30 solar masses merging with each other and producing a merger remnant,

63
00:07:00,520 --> 00:07:03,670
which is higher than 50 solar masses in many cases.

64
00:07:04,000 --> 00:07:12,219
There are some where you have lower mass black holes of a few solar masses merging with others, and you also have neutron stars merging,

65
00:07:12,220 --> 00:07:21,070
which are have masses of 1.4 solar masses, roughly speaking, and they produce something which we don't know exactly what they are.

66
00:07:21,070 --> 00:07:23,979
But in future, in the future versions of these instruments,

67
00:07:23,980 --> 00:07:29,680
you can see whether or not they are black holes or it's some kind of a of a exotic neutron star.

68
00:07:29,710 --> 00:07:33,550
For example, there are these other points in this in this slide,

69
00:07:34,000 --> 00:07:42,610
which would show actually the electromagnetic previous electromagnetic observations of both neutron stars and black holes.

70
00:07:42,720 --> 00:07:51,910
It turns out if a black hole is orbiting around the star it studies orbiting around the black hole, the star can can see the black hole.

71
00:07:51,910 --> 00:07:58,410
And so the gas flowing into the black or shines. And you can detect it with electromagnetic radiation and infer the presence of the black hole.

72
00:07:58,420 --> 00:08:05,979
So this was this was done before these gravitational wave detection were made, but they found lower masses.

73
00:08:05,980 --> 00:08:09,730
As you can see, these black holes often have much higher masses.

74
00:08:09,740 --> 00:08:17,720
So this is already something interesting. A new. And so by measuring these signals, you can learn about the parameters describing.

75
00:08:17,770 --> 00:08:22,150
So here's a long list of all of these signals and some of the parameters that you can measure.

76
00:08:22,510 --> 00:08:28,870
So you can measure the masses, as I said, of both objects. But it's not a very accurate measurement.

77
00:08:28,900 --> 00:08:37,390
There are some error bars, and this is showing the confidence like distribution function of the inferred mass for the two objects.

78
00:08:37,720 --> 00:08:41,860
So in some cases they are narrow and in others they are quite poorly constrained.

79
00:08:42,490 --> 00:08:49,330
You can also infer the mass ratio from these parameters, which are often poorly constrained, as you can see.

80
00:08:50,350 --> 00:08:54,900
But you can also measure the spins. So the black holes are spinning.

81
00:08:54,940 --> 00:08:58,540
They have angular momentum. This progenitor star has angular momentum.

82
00:08:58,540 --> 00:09:01,630
So you expect the black holes to perhaps they're spinning.

83
00:09:02,050 --> 00:09:06,610
And you can measure one component of the spin, which is the component.

84
00:09:06,820 --> 00:09:13,420
So spin is normally a vector, but the component along the angular momentum to the direction of the orbital axis,

85
00:09:13,420 --> 00:09:20,889
you can measure the spins along that in that direction relatively accurately, as you can see with some large error bars.

86
00:09:20,890 --> 00:09:24,730
But you can see a magnet and you can detect the distance to the source.

87
00:09:24,910 --> 00:09:33,030
So this is basically all you know. And so then you can look at these distributions, what comes out, but what did the instruments show?

88
00:09:33,810 --> 00:09:43,080
And so this is, for example, you can plug the mass distribution of these detections and you can see that this is what they get.

89
00:09:43,860 --> 00:09:47,670
And so this is based on 100 events that were detected or 90 events that.

90
00:09:48,860 --> 00:09:57,260
You can see that there are lots of events with masses between, you know, up to ten stolen masses.

91
00:09:57,620 --> 00:10:01,699
But this distribution extends all the way to 70 and actually beyond.

92
00:10:01,700 --> 00:10:07,670
Now they're newer detections extending to even beyond the 100 similar masses.

93
00:10:08,420 --> 00:10:10,820
As I will show you, this is somewhat surprising already.

94
00:10:11,030 --> 00:10:19,250
So they are somehow very massive black holes in the universe, which we didn't know of previously, which are actually merging, which is interesting.

95
00:10:20,430 --> 00:10:31,290
And so you can you can also plot the mass distribution of the primary black hole mass as a function of like the density of them in the universe,

96
00:10:31,290 --> 00:10:40,740
the merger density in the universe. And again, you see that this is peak that there and then but the distribution extends to the larger masses.

97
00:10:41,840 --> 00:10:46,270
So the heavier ones are easier to see because they they produce stronger signals.

98
00:10:46,280 --> 00:10:51,980
So that's why the mass distribution I showed you here extended to two higher masses.

99
00:10:52,310 --> 00:10:58,170
So we see more of the massive ones. And you can also see that spins.

100
00:10:58,800 --> 00:11:04,140
So how quickly the black was spinning and there's a maximum spin that the black horse can have.

101
00:11:04,680 --> 00:11:11,730
So this is this is so the spins are usually normalised to the maximum.

102
00:11:12,300 --> 00:11:22,110
And so this is if they are, they have spin one that means they are maximally spinning in that case and that the geometry

103
00:11:22,110 --> 00:11:28,679
is basically swirling around the black hole at almost the speed of light at the horizon.

104
00:11:28,680 --> 00:11:33,510
And if the black holes were spinning even more rapidly than you would see a naked singularity.

105
00:11:34,080 --> 00:11:37,770
And there's a conjecture that that these types of blackness does not exist.

106
00:11:38,190 --> 00:11:48,210
And if you collapse a star into a black hole, you never form anything above unity, spin in this in these units.

107
00:11:48,930 --> 00:11:53,130
And so so you can you can see what the observations show.

108
00:11:53,370 --> 00:11:55,220
And they seem to cluster and zero.

109
00:11:55,230 --> 00:12:05,190
So the so the spin component in along the orbital axis seems to be close to zero, which is somewhat surprising, as I will show you.

110
00:12:05,790 --> 00:12:10,440
And you can look at possible the correlations between these these parameters.

111
00:12:10,450 --> 00:12:16,470
Is there correlation between mass spins for individual sources? But there is no clear trend that you see.

112
00:12:18,270 --> 00:12:22,410
You can also look at the rate of at which the black was merged in the universe.

113
00:12:22,770 --> 00:12:29,670
You can you could already measure this with the first two detections that were announced in 2016.

114
00:12:30,180 --> 00:12:33,690
And the merger rates at that point were quite poorly constrained.

115
00:12:33,690 --> 00:12:38,760
It was between two and 600 per gigabyte. Q Per year in the universe.

116
00:12:39,120 --> 00:12:43,410
But there's more and more detections came along. The error bars shrunk.

117
00:12:44,100 --> 00:12:54,660
And so at the at present, and it's even better known now, it's around 30 gigabytes per year.

118
00:12:55,350 --> 00:12:58,590
And there's there were also detections of neutron star neutron star mergers.

119
00:12:58,890 --> 00:13:04,980
So there are now three detections, as you can see.

120
00:13:04,980 --> 00:13:09,389
And that and the merger rate is quite poorly known.

121
00:13:09,390 --> 00:13:13,890
But it's it's it seems to be much higher than the rate of black hole backflip mergers.

122
00:13:13,920 --> 00:13:17,580
This is not surprising because there are more neutron stars in the universe than black holes.

123
00:13:19,100 --> 00:13:21,580
But they are only more by a factor of three.

124
00:13:21,970 --> 00:13:29,890
So it's somewhat possibly somewhat surprising if if this number is much higher than if you compare to the or black or.

125
00:13:30,580 --> 00:13:36,550
There's also examples of black hole neutron star coalescence is that you can see in the future.

126
00:13:36,910 --> 00:13:44,680
You can also make plots of the merger rate how it evolves with Redshift so the distance from us.

127
00:13:45,190 --> 00:13:52,300
So Redshift as the universe expands, describes the distance to the sources and the cosmological scale.

128
00:13:52,780 --> 00:14:00,850
And you can you can basically measure how the merger rate changed in time as the universe evolved.

129
00:14:01,990 --> 00:14:05,620
And you can contrast that theories and try to learn about the sources.

130
00:14:06,840 --> 00:14:12,900
And they are future extensions of the present instruments.

131
00:14:13,710 --> 00:14:22,500
And so currently we have legal, but in the future we will have like or A-plus is a advanced version of the advanced legal instrument.

132
00:14:23,010 --> 00:14:26,340
And and there are different instruments being planned on earth.

133
00:14:26,760 --> 00:14:34,290
And this will reduce the noise in these instruments so you can increase the sensitivity to larger and larger redshifts.

134
00:14:34,860 --> 00:14:45,509
So when these future instruments come along, maybe in ten years you will see basically all of the mergers to do almost 100 redshift,

135
00:14:45,510 --> 00:14:49,620
which is even beyond the time at which the first stars formed.

136
00:14:49,860 --> 00:14:58,320
So you can see and most of the mergers in the universe, at least at these mass in the in these massive changes.

137
00:14:58,740 --> 00:15:01,380
And so based on the rate of mergers that we already know,

138
00:15:01,950 --> 00:15:08,020
we can calculate how many mergers there are in the universe in a in this volume that we can detect.

139
00:15:08,040 --> 00:15:11,910
So at the moment, we are limited to redshift of 0.5.

140
00:15:12,510 --> 00:15:18,900
So there are one, two, three mergers per day, but we don't see all of them because the instruments are not sensitive to all directions equally.

141
00:15:19,500 --> 00:15:25,500
And but this is the order that we see there, rates that we see it at present.

142
00:15:26,010 --> 00:15:33,080
But when we see it shifts up to two and we will see a couple of mergers per hour.

143
00:15:34,240 --> 00:15:39,660
Based on this rate. So we can we can start asking questions about the sources.

144
00:15:39,700 --> 00:15:48,700
So what what are we seeing? So the big questions are how the black hole has form and how did the binary form?

145
00:15:49,240 --> 00:15:56,350
Why are black holes in binaries and why do they merge? Why do they come so close that gravitational waves can actually merge them?

146
00:15:56,920 --> 00:16:02,379
Turns out this is this is this is an interesting question because you need to put the black was very,

147
00:16:02,380 --> 00:16:08,420
very close such that the gravitational waves actually merge the black hole is otherwise it takes too long.

148
00:16:08,440 --> 00:16:16,360
So if if the the black holes are at the typical separation where we see stars to be in binaries,

149
00:16:17,140 --> 00:16:21,910
they will only merge in much larger time than the than the age of the universe.

150
00:16:22,390 --> 00:16:28,300
So in order for them to merge, you have to put them at a distance, which is less than the earth from the sun.

151
00:16:28,720 --> 00:16:39,170
You have to put two black holes, the masses of tens or 30 solar masses at on the scale of the mercury through the sun such that they actually merge.

152
00:16:39,190 --> 00:16:44,860
But what is the process which takes them to such small distances so that they can merge the two gravitational waves?

153
00:16:46,260 --> 00:16:50,400
And so the big question is what are the most likely environments for mergers?

154
00:16:51,030 --> 00:16:55,200
Do mergers happen in the in the galaxy, in the galactic disk?

155
00:16:55,620 --> 00:16:57,599
Do they happen in the galactic bulge,

156
00:16:57,600 --> 00:17:05,460
the central regions of the galaxy or star clusters like globular clusters or the halo which surrounds the galaxy, as I recall.

157
00:17:06,450 --> 00:17:11,159
So these are some of the possibilities and let me just take you through some

158
00:17:11,160 --> 00:17:17,310
of them and the big problems that each of these areas are face at the moment.

159
00:17:18,120 --> 00:17:21,330
So first question is how do black holes form a binary?

160
00:17:21,780 --> 00:17:29,550
So, so the simplest idea is that we already see stars to be in binaries, often especially massive stars.

161
00:17:29,910 --> 00:17:35,250
And we know that black holes form from massive stars, lower mass stars form white dwarfs typically,

162
00:17:35,640 --> 00:17:42,000
and higher mass stars form neutron stars, and only the highest mass stars form black holes.

163
00:17:42,570 --> 00:17:46,800
But these higher, highest mass stars are often observed in binaries.

164
00:17:47,520 --> 00:17:55,590
And so this is these are observed. And and so you can you can you can you can ask what happens with these binaries at late banks.

165
00:17:55,980 --> 00:18:01,350
So eventually the stars run out of hydrogen and they expand, they become red giants.

166
00:18:01,890 --> 00:18:06,390
And at that point they can transfer mass to the other star.

167
00:18:06,870 --> 00:18:11,850
And ultimately the star explodes in a supernova explosion and turns into a black hole.

168
00:18:12,420 --> 00:18:15,750
So then you have a black hole and another star surrounding it.

169
00:18:16,350 --> 00:18:23,370
And this black hole can actually suck matter from the other star and shine in X-rays.

170
00:18:23,700 --> 00:18:27,210
So this gas, which is flowing in the direction of the black hole, is observable.

171
00:18:27,570 --> 00:18:34,200
So you can observe this stage of the evolution. And then there's this star also explodes as a supernova.

172
00:18:34,530 --> 00:18:38,190
It forms another black hole. And so you have a black hole. Black hole binary.

173
00:18:39,180 --> 00:18:46,380
But there are open questions. So but let me just show you some some of this in more detail.

174
00:18:46,890 --> 00:18:51,610
So this is an example for what happens if you start with the black holes,

175
00:18:51,610 --> 00:18:58,349
the water mass of 100 or 90 and 60 stories is then ultimately this one of the

176
00:18:58,350 --> 00:19:05,760
stars expands and stands for some mass and then it turns into a black hole.

177
00:19:06,390 --> 00:19:09,690
And and then the other star expands.

178
00:19:10,080 --> 00:19:14,400
And there's a phase when this star is actually engulfing the black hole.

179
00:19:14,520 --> 00:19:25,200
So it expands. Red giant becomes so large that it actually engulfs the other objects that the black hole is now moving inside the other star.

180
00:19:25,980 --> 00:19:29,520
And ultimately the star turns into a black hole.

181
00:19:30,090 --> 00:19:35,940
And and then you have two black holes at short distances, which then merge into gravitational waves.

182
00:19:36,570 --> 00:19:39,570
And so you see that there are many stars in each galaxy.

183
00:19:39,780 --> 00:19:44,480
Then for the 11 stars, let's say, and we know what fraction of them are massive.

184
00:19:44,490 --> 00:19:50,700
And so all of them turn into black holes. And so we have an estimate of how many black holes we should have in each galaxy.

185
00:19:51,000 --> 00:19:56,250
And it's quite a large number, 10 million to 100 million black holes in each galaxy.

186
00:19:56,850 --> 00:20:02,010
But the question is how many of them are in binaries? Let's say most of them because massive stars are in binaries.

187
00:20:02,340 --> 00:20:06,959
But how how many of them are actually in such a closed binary that they have these

188
00:20:06,960 --> 00:20:10,530
carbon envelope phase that they can actually go so close that they can eventually,

189
00:20:10,560 --> 00:20:16,230
which is an open question. And and most massive stars are in wide binaries.

190
00:20:16,320 --> 00:20:25,830
So that that that helps. And some of them are in theatres so that there may be a third star which is which helps the merge the intervening.

191
00:20:26,800 --> 00:20:30,190
But so there are some open questions with this theory.

192
00:20:30,760 --> 00:20:36,430
And here are some of them. So basically, all of the all of the points I've laid out here.

193
00:20:37,910 --> 00:20:42,170
Are poorly known. So there are so many open questions.

194
00:20:42,180 --> 00:20:46,640
There are basically three parameters in all of these theories.

195
00:20:47,540 --> 00:20:52,459
And and you're constraining these three free parameters with observations.

196
00:20:52,460 --> 00:21:00,040
But then the question is, if you have so many free parameters, what can you actually learn from these types of observations?

197
00:21:00,470 --> 00:21:05,720
And so you can see that the biggest question is what happens during the common envelope evolution?

198
00:21:05,720 --> 00:21:11,150
This is poorly understood. What happens to an object inside another star, for example?

199
00:21:11,900 --> 00:21:18,140
And the only point where you don't have question marks is the last point that we have two black holes.

200
00:21:18,410 --> 00:21:24,380
It's move, you know, in response to Einstein's equations, and they emerge because of gravitational waves.

201
00:21:24,470 --> 00:21:28,560
We only know this part. But so what can what can we do?

202
00:21:28,580 --> 00:21:31,880
How can we make progress? And we can look at the observations.

203
00:21:31,910 --> 00:21:38,570
So as I told you, you can observe the part of this evolution when there's a star and a black hole next to it.

204
00:21:38,930 --> 00:21:47,870
And so these are X-ray binaries, and you can measure the masses of these X-ray binaries and they turn out to be clustered into two categories.

205
00:21:48,370 --> 00:21:56,030
One is around 1.4 solar masses. These are believed to be neutron stars and others are around ten solar masses.

206
00:21:56,060 --> 00:22:01,380
These are believed to be black holes. And so what we see is the gas flowing into them.

207
00:22:02,250 --> 00:22:08,790
And based on the spectra, you can infer that these masses and you can also look at the spins.

208
00:22:09,570 --> 00:22:15,680
And so but here so the big question is, why are the merging black holes that Ligo's see so massive?

209
00:22:15,690 --> 00:22:21,210
It's unclear. Especially if you think about stellar evolution.

210
00:22:21,930 --> 00:22:28,979
It turns out that the prediction, the theoretical prediction is that you should not see any black holes with mass

211
00:22:28,980 --> 00:22:32,730
higher than 50 solar masses because they just cannot form from stellar evolution.

212
00:22:32,940 --> 00:22:40,770
If you have a very, very massive star, it will typically shed most of its mass and so it will just lose its mass in vain.

213
00:22:40,780 --> 00:22:50,929
So there's a solar wind coming out of these massive stars and they they lose so much mass that the remnant is typically smaller than 50 solar masses.

214
00:22:50,930 --> 00:22:59,130
If it if it was bigger and there's an explosion as instability and an explosion which just pushes the star apart.

215
00:22:59,620 --> 00:23:11,470
And there's this these pairs form these particle antiparticle pairs form and and you expect to see no, no black hole left back with higher mass.

216
00:23:11,490 --> 00:23:17,940
So so this is an open question why we see black holes with so high mass in in the observations.

217
00:23:19,070 --> 00:23:22,250
And the other question is, why are the black holes not spinning?

218
00:23:22,850 --> 00:23:28,310
Because, first of all, theoretically, you expect them to be spinning because if you have a star in a black hole next to it,

219
00:23:29,090 --> 00:23:32,390
mass is flowing into the black hole which spins up in the black hole.

220
00:23:32,840 --> 00:23:37,110
And the other thing is that the black hole is orbiting the star, which spins of the Star.

221
00:23:37,130 --> 00:23:40,970
These objects are very, very close. So they have strong tidal interactions.

222
00:23:41,210 --> 00:23:49,820
And so you expect to see very highly spinning stars, which should conserve their angular momentum as they collapse through a black hole.

223
00:23:50,180 --> 00:23:53,750
And so you expect to have very highly spinning black holes in the end.

224
00:23:54,350 --> 00:24:03,750
And indeed, the X-ray observations have shown that there are only three objects which have each are progenitors of black hole, black hole mergers.

225
00:24:03,770 --> 00:24:10,670
So these are the so-called high mass X-ray binaries where the donor star will turn into a black hole and not a neutron star.

226
00:24:11,000 --> 00:24:19,400
And all of these observations indicate that the spin is large for these for these black holes,

227
00:24:19,970 --> 00:24:26,629
but the gravitational waves so that the merging black holes are actually not spinning highly.

228
00:24:26,630 --> 00:24:33,560
So this is another indication that maybe this is not the possibly not the correct theory explaining the sources.

229
00:24:35,110 --> 00:24:38,170
Okay. So is there anything, any other idea around?

230
00:24:38,340 --> 00:24:43,600
Well, there are. There's a very famous one called the Dynamical Merger Channel.

231
00:24:43,990 --> 00:24:51,790
And here the idea is that you have globular clusters or galactic nuclei which host very large population of stars in a very small region.

232
00:24:52,090 --> 00:25:03,070
So you have these globular clusters where you have roughly a million stars in a very small region of one parsec, one light, a few light years.

233
00:25:03,700 --> 00:25:06,879
And so you have a million stars, whereas in the galaxy,

234
00:25:06,880 --> 00:25:16,600
the closest are through to earth and to the sun is the next one is is the same is a few light years away.

235
00:25:16,900 --> 00:25:24,970
But here you have a million stars in that region. And so in this case, you have a possibility of close encounters between these stars.

236
00:25:25,180 --> 00:25:30,640
So you can form binaries dynamically as stars approach each other, or three stars come by.

237
00:25:30,650 --> 00:25:32,860
They have a complicated gravitational interaction,

238
00:25:33,250 --> 00:25:40,900
and one of the stars is ejected and the leftover stars form as a stellar binary, which can then merge.

239
00:25:41,410 --> 00:25:49,450
And there are further interactions with other stars, and this can reduce their separations to the scale we need for gravitational waves to merge them.

240
00:25:50,050 --> 00:25:54,459
And then galactic nuclei are even denser environments so that these very centres of galaxies,

241
00:25:54,460 --> 00:26:05,440
you have supermassive black holes and in that region you have the density up to 1000000 to 1000000000 times larger than indigo in the field.

242
00:26:05,860 --> 00:26:10,810
And so the possibly the rate of merger will be higher in that region.

243
00:26:12,550 --> 00:26:25,060
And so you can calculate what is the encounter rate between objects and which which come close and which can shrink the binary.

244
00:26:25,090 --> 00:26:31,480
Once you have a binary and this turns out to be a scattering process which is proportional to the square of the density.

245
00:26:32,260 --> 00:26:39,400
And in this region, the scale of the density is ten to the 12 to 10 to the 20 times higher than in the galactic sphere.

246
00:26:39,430 --> 00:26:43,000
So here this process can actually happen and you can simulate that.

247
00:26:43,420 --> 00:26:49,870
So this is a theoretically clean problem. Not, not, not like stellar evolution that I showed you,

248
00:26:50,200 --> 00:26:54,670
because here you just have point masses moving around that you can put on a computer and see what happens.

249
00:26:55,570 --> 00:26:59,900
And so here is a simulation. And.

250
00:27:01,120 --> 00:27:07,719
And so this shows that black holes are moving around and stars actually in simulations.

251
00:27:07,720 --> 00:27:10,300
I think they are only stars, but in others they are black holes.

252
00:27:10,780 --> 00:27:16,749
And it turns out that the central parts at the centre, the massive stars segregate in the centre,

253
00:27:16,750 --> 00:27:20,410
they form binaries and they start ejecting other stars that they encounter.

254
00:27:20,830 --> 00:27:25,990
And ultimately the cluster loses a lot of many stars because of this.

255
00:27:26,410 --> 00:27:32,590
And, and eventually in the in the simulations, the cluster disperses completely.

256
00:27:33,040 --> 00:27:35,590
So these star clusters are a transient phenomena.

257
00:27:35,620 --> 00:27:43,360
They should not last forever, but luckily and deliver the early time in the age of the universe that these are still around.

258
00:27:43,870 --> 00:27:51,940
So you can still use them as probes of of of of astrophysics and and they provide a channel for gravitational wave sources.

259
00:27:52,330 --> 00:27:56,110
So there are many processes that happen. So there's a dense population.

260
00:27:56,380 --> 00:28:00,220
There's a possibility for three objects to come close.

261
00:28:00,370 --> 00:28:04,540
And so there's a scattering encounter which forms the binary.

262
00:28:04,540 --> 00:28:10,870
And then there are these binary single interactions and binary interactions, the massive objects seen through the centre.

263
00:28:11,230 --> 00:28:16,900
And eventually you have memories which come so close that they emerge because of gravitational waves.

264
00:28:17,140 --> 00:28:23,860
So this is the ADF and you can use these theories and simulations to make predictions for what should be the rate of mergers,

265
00:28:24,190 --> 00:28:30,070
what should be the mass distribution of mergers, that spinning the distributions, and you can then check with the observations.

266
00:28:31,100 --> 00:28:35,270
And. It turns out there are some problems.

267
00:28:35,280 --> 00:28:39,990
But let me just show you how this may look like in nature if you just looked at it.

268
00:28:40,470 --> 00:28:47,910
So this here at the very centre, there's a there are black holes flying around at the centre of the globular cluster.

269
00:28:48,120 --> 00:28:52,980
They seem to, because they are more massive than stars and they distort the light from,

270
00:28:53,460 --> 00:28:57,240
from the background, stars and arrows of stars flying around this region.

271
00:28:57,840 --> 00:29:08,010
And then and so it's quite a dynamic environment and you form binaries and eventually they merge and by

272
00:29:08,070 --> 00:29:14,940
studying gravitational waves you finally can look in this region and see and detect signals from these.

273
00:29:15,390 --> 00:29:20,670
These are highly dynamic, dark regions of spacetime.

274
00:29:23,500 --> 00:29:29,970
Okay, So. So what is the prediction for spins? So the implication is that it should be centred.

275
00:29:30,060 --> 00:29:40,049
Well, this is the observation. So there. So the prediction is that if you have an isotropic distributed cluster, so it's based on the assumption,

276
00:29:40,050 --> 00:29:45,090
but you assume for the initial spin distributions, but if the spins say it's a spherical cluster,

277
00:29:45,090 --> 00:29:51,479
you can assume that it's a isotropic distributed cluster and that there's no particular

278
00:29:51,480 --> 00:29:55,220
reason why you should expect this things to be in any particular direction.

279
00:29:55,230 --> 00:29:57,870
So it's the assumption is that in globular clusters,

280
00:29:57,870 --> 00:30:04,860
the spins of these black holes may be arbitrarily isotropic and you form binaries back there in close encounters,

281
00:30:05,280 --> 00:30:09,540
which do not synchronise these spins.

282
00:30:10,080 --> 00:30:17,219
So you can have these black or black or binaries, and there are these exchange interactions when black holes can pick out stars from an

283
00:30:17,220 --> 00:30:21,450
existing binary and sit in their place and then another black can kick out the other star.

284
00:30:21,840 --> 00:30:25,710
So you can form black or black or binary with an arbitrary spin direction.

285
00:30:26,250 --> 00:30:35,250
And in this case the component along the orbital angular axis, which is the component that is measured, will be distributed around zero.

286
00:30:35,310 --> 00:30:38,490
So this is the prediction and it matches the observations.

287
00:30:38,490 --> 00:30:45,600
So that's so far so good. But the rate of mergers is the prediction is too low.

288
00:30:46,080 --> 00:30:52,200
So if you look at the simulations and so they are Monte Carlo Markov chain and

289
00:30:52,200 --> 00:30:57,570
direct and but it's information that I just showed you and these simulations

290
00:30:57,780 --> 00:31:05,800
is a complicated simulations run run them on a supercomputer and get that the rate of mergers in the universe should be around six per gigabyte.

291
00:31:06,000 --> 00:31:12,700
You pretty? But the observed rate is between 15 and 39 gigabytes 60 per year, so it's a bit lower.

292
00:31:13,540 --> 00:31:19,490
So why is that? Is it is it possible that you just made some error in the numerical code?

293
00:31:19,570 --> 00:31:24,240
This is the first thing that comes to mind. But actually, this number makes sense.

294
00:31:24,250 --> 00:31:30,969
So you can you can understand this. So if you assume that each black hole merges at most once.

295
00:31:30,970 --> 00:31:35,560
So this is an upper limit. Let's say all of the black was merged in the cluster.

296
00:31:36,040 --> 00:31:41,769
And you can calculate how many black holes you have in the cluster because you see how many stars you have and you know,

297
00:31:41,770 --> 00:31:46,840
what is the mass distribution of stars and you know, how many of them will turn into black holes.

298
00:31:47,200 --> 00:31:53,170
So you know how many black you should have in a cluster and you see how many globular clusters you have in the universe.

299
00:31:53,530 --> 00:31:58,450
And if each of them merges once in the age of the universe, the present stage of the universe, you get a rate.

300
00:31:59,430 --> 00:32:05,040
And this rate turns out to be 40. So this is an upper limit for the past couple years.

301
00:32:05,070 --> 00:32:14,450
So if all of them are but the simulations show that only 20% of them merge and only about 10 to 20% of them actually form binaries.

302
00:32:14,460 --> 00:32:18,610
So most of them are single and only half of the banks merge.

303
00:32:18,630 --> 00:32:25,370
So the rate of merger is a factor of ten lower, so you get from 49 to 4.

304
00:32:25,380 --> 00:32:29,820
But this is just the rough estimate. So actually it's around six, but you understand where this comes from.

305
00:32:30,450 --> 00:32:34,230
And still so this is an upper upper limit type estimate.

306
00:32:35,310 --> 00:32:39,900
So so it's not clear If so, you might be missing most of the mergers here.

307
00:32:40,000 --> 00:32:44,140
This is the bottom line. And you can make the for the mass distributions.

308
00:32:44,660 --> 00:32:48,620
It turns out that more massive black holes emerge more easily in these systems.

309
00:32:49,250 --> 00:32:54,050
And you can plot to the expectations as a function of mass ratio and total mass.

310
00:32:54,410 --> 00:32:57,890
And most of them will be merging at high masses.

311
00:32:58,070 --> 00:33:02,780
So those are the ones which sink to the centre and have the most encounters with other objects.

312
00:33:03,410 --> 00:33:08,420
And there's interestingly a small population here with even higher masses.

313
00:33:09,290 --> 00:33:14,810
So so you form black holes with masses lower than 50 solar masses.

314
00:33:15,020 --> 00:33:23,330
And let's say these merge over here. So they have a mass ratio of around 1 to 50 smaller mass that holes the total mass 100 at maximum.

315
00:33:23,900 --> 00:33:30,380
But once they merge, they form a remnant. And this remnant, if it's not kicked out from the cluster, can merge again.

316
00:33:31,040 --> 00:33:34,940
And so you can predict what is the fraction of population which merges again.

317
00:33:35,480 --> 00:33:44,000
So these are the second generation mergers, and these are possible in globular clusters if the progenitor blackhole does not seem rapidly.

318
00:33:44,450 --> 00:33:48,410
And in this case, 5 to 10% would be second generation mergers.

319
00:33:48,860 --> 00:33:54,350
Third generation mergers are difficult to produce in globular clusters because that would that merge.

320
00:33:54,800 --> 00:33:57,290
They experience a gravitational wave kick.

321
00:33:57,410 --> 00:34:04,280
So the emission of gravitational waves is not isotropic and there's a lot of energy carried away by gravitational waves.

322
00:34:04,700 --> 00:34:14,150
As Steve mentioned, there's three solar masses, M.C. Squared energy carried away in in not exactly isotropic.

323
00:34:14,570 --> 00:34:21,410
If some of it is carried a is is ejected in one direction is emitted in my direction.

324
00:34:21,620 --> 00:34:28,280
Then the black hole will be kicked in the opposite direction. And so there's this kick that the remnant experiences.

325
00:34:28,820 --> 00:34:34,250
And in these environments, then the gravitational veil is relatively shallow.

326
00:34:34,490 --> 00:34:38,660
You ask the escape velocity from these globular clusters is relatively low.

327
00:34:38,660 --> 00:34:46,250
So you expect some of these black was to be objective. And if the black hole is actually merge, they produce highly spinning black holes.

328
00:34:46,670 --> 00:34:52,640
And the this if you have a highly spinning black hole, then this kick velocity will be much larger.

329
00:34:52,970 --> 00:34:58,820
So in that case, we expect all of them to be ejected and not merge again in this cluster.

330
00:35:01,000 --> 00:35:06,130
And so this is the question how can Black was merge multiple times and not get ejected?

331
00:35:06,460 --> 00:35:15,710
So it turns out that the measurements indicate that 83 events are one G plus first generation mergers.

332
00:35:15,710 --> 00:35:24,220
So they are consistent with ordinary theory. But five of the events are of this type one G plus two G,

333
00:35:24,760 --> 00:35:32,350
meaning that a second generation black hole is was not ejected and is merging again with another black hole in the cluster.

334
00:35:32,990 --> 00:35:38,620
And this is consistent with the rate being 5% for these ones.

335
00:35:39,010 --> 00:35:50,590
But you also see two G plus two G mergers, two events which are not clear if that happens or higher than the recent events in in globular clusters.

336
00:35:51,770 --> 00:35:55,790
And also, by the way, done. Some of the murderers have eccentricity.

337
00:35:56,360 --> 00:36:04,790
It turns out that the eccentricity distribution should have three peaks for these types of black holes,

338
00:36:04,790 --> 00:36:12,980
which are binaries which form in globular clusters. So there are binaries which which are merging outside of the globular cluster.

339
00:36:13,010 --> 00:36:19,160
They are binaries which merge in the globular cluster and F due to gravitational waves,

340
00:36:19,610 --> 00:36:23,810
and they are binaries which merge during the encounter of three objects.

341
00:36:23,840 --> 00:36:28,680
So that also happens. And it turns out that those are those mergers.

342
00:36:28,700 --> 00:36:35,480
So the ones which which happened during the three by encounter three black holes encountering each other are 5% of all of them.

343
00:36:36,470 --> 00:36:42,470
And and these ones should be eccentric. In that case, they they form binaries.

344
00:36:42,470 --> 00:36:49,549
The merging binary is such a small separation that there's not enough time for the signal for the orbit disaccharides before they merge.

345
00:36:49,550 --> 00:36:56,360
So these should be eccentric. And so this would detect 5% of these globular cluster mergers to be eccentric.

346
00:36:56,810 --> 00:37:04,430
At the moment there's only one detection which is even controversial, which may possibly be eccentric out of 100.

347
00:37:04,820 --> 00:37:12,530
So but in the near future you will have many more sources so you can test this paradigm with eccentricity observations.

348
00:37:14,270 --> 00:37:16,880
And finally, there's the question about the dark matter halo.

349
00:37:17,480 --> 00:37:30,050
So you have the galaxy surrounded by this mysterious massive dark component, which you can infer based on the motion of stars in the galaxy.

350
00:37:30,710 --> 00:37:39,470
And the interesting idea, which came up after the first discovery was if there is dark matter was actually made of black holes,

351
00:37:40,250 --> 00:37:48,140
then it would be nice if if, if this was the case, because then if black holes merge, if these dark matter black holes merge,

352
00:37:48,650 --> 00:37:52,160
then you can actually see them with gravitational waves. They're not dark anymore.

353
00:37:53,180 --> 00:37:58,550
And so there were some observational probes earlier with electromagnetic observations.

354
00:37:59,540 --> 00:38:08,630
So if you have lots of these black holes in the dark matter, then you should see lensing variable light from from different sources.

355
00:38:09,110 --> 00:38:13,100
And this puts a constraint on the masses of these types of black holes.

356
00:38:13,370 --> 00:38:16,820
You also see binaries in the binaries in the galaxy.

357
00:38:17,000 --> 00:38:24,380
It would be disruptive if you would have these massive dark matter particles, black holes flying around in the in the galaxy.

358
00:38:24,410 --> 00:38:28,070
But we see these binaries, so that also puts a limit on of black holes.

359
00:38:28,550 --> 00:38:34,190
But there's a range which is still allowed, which could be the ones that losses.

360
00:38:34,250 --> 00:38:38,440
So that's very interesting that maybe this is just dark matter, but this you.

361
00:38:39,940 --> 00:38:45,610
And in fact, there was a paper which claimed that if 100% of the dark matter is made up, 30 cylinders,

362
00:38:45,970 --> 00:38:51,070
single black holes, and they should produce exactly the same rates that we see that Lagos is.

363
00:38:51,280 --> 00:38:54,370
So this is a coincidence, which is very interesting.

364
00:38:55,000 --> 00:39:00,969
But there are other studies which showed that that there was a miscalculation.

365
00:39:00,970 --> 00:39:08,950
And if you make it make a more accurate estimate, then only 1% of the dark matter is sufficient to explain all of the detections.

366
00:39:09,250 --> 00:39:14,890
And if 100% of the dark matter is made of black holes, then you overproduce the mergers that you see.

367
00:39:15,700 --> 00:39:24,399
And you can also have black holes in principal in the early universe that no one knows if primordial black holes existed,

368
00:39:24,400 --> 00:39:29,710
that there after shortly after the Big bang. But you can prove them with gravitational waves now.

369
00:39:30,280 --> 00:39:37,689
And if only 0.1% of dark matter is in primordial binary black holes after inflation in the early universe,

370
00:39:37,690 --> 00:39:41,680
then those then only those will explain all of the mergers.

371
00:39:42,130 --> 00:39:45,790
So that's very interesting. Of course, it's a little bit speculative.

372
00:39:46,300 --> 00:39:51,360
So I think it's still basically an open question.

373
00:39:51,370 --> 00:39:58,929
So this is the summary. So these are the possible ideas and possible problems that there are.

374
00:39:58,930 --> 00:40:01,960
And so there are some other ideas that I didn't have time to go into.

375
00:40:02,320 --> 00:40:06,820
So, for example, the third, third star around the binary can have merged this binary.

376
00:40:07,330 --> 00:40:12,640
But again, the problem here is maybe there is not enough of them in the right configuration for them to merge.

377
00:40:13,300 --> 00:40:17,240
And the other idea is that maybe in galactic nuclei you have a supermassive black hole.

378
00:40:17,260 --> 00:40:22,170
In that case, supermassive black hole helps to to increase the escape velocity.

379
00:40:22,180 --> 00:40:27,820
So in that case, it's much harder to eject the object. So you will have multiple generations of mergers more easily.

380
00:40:28,270 --> 00:40:32,679
So maybe, maybe, maybe this will be the the answer.

381
00:40:32,680 --> 00:40:40,260
But it's at the moment it's not clear. And dark matter Halo is another possibility, but it's somewhat of an exotic possibility.

382
00:40:41,370 --> 00:40:48,570
And I think I'm out of time. I just want to highlight another idea that we've been pursuing here at Oxford with some brilliant students.

383
00:40:49,050 --> 00:40:54,230
So here we are investigating if there's a supermassive black hole at the centre of the galaxy.

384
00:40:54,240 --> 00:41:01,560
In some cases there's a gas disk surrounding the black hole and the gas can help to merge the stellar mass black holes in this environment.

385
00:41:01,590 --> 00:41:06,200
So you have a star cluster which is observed a supermassive black hole.

386
00:41:06,270 --> 00:41:14,129
Again, you can calculate how many of them will turn into into black holes and you expect something like 10000 to 20000 stellar mass black holes,

387
00:41:14,130 --> 00:41:18,900
small black holes to, you know, orbit around a big supermassive black hole.

388
00:41:19,170 --> 00:41:24,570
In this big supermassive black hole. Keeps these small that close in there in its vicinity.

389
00:41:25,380 --> 00:41:27,930
And there's also gas flowing into the supermassive black hole,

390
00:41:27,930 --> 00:41:34,490
which shines brilliantly and spectacularly that we can see from the other side of the universe.

391
00:41:34,500 --> 00:41:41,010
So this is observed. And so the prediction is that there should be stellar mass black, which then interacts with the gas cloud.

392
00:41:41,820 --> 00:41:48,930
And so you expect to see black holes get captured in this gas clouds in relatively short time.

393
00:41:48,960 --> 00:41:57,060
Within ten years, you can calculate this and you can calculate what happens with objects in gas and they emerge relatively quickly because of gas.

394
00:41:57,300 --> 00:42:03,870
So then you solve this problem of how do you bring the black holes close with the gas dissipation.

395
00:42:05,110 --> 00:42:11,150
And and so the interesting thing here is that you have the possibility of having an electromagnetic counterpart,

396
00:42:11,180 --> 00:42:17,430
the signal, because the object is not just in in darkness, but there's gas around.

397
00:42:17,450 --> 00:42:21,880
So you might excite the gas and produce some kind of a flare.

398
00:42:22,320 --> 00:42:31,360
In fact, one of the events that we observed had an energy and flare in the same region of the sky at the same time and 34 days after the event.

399
00:42:31,360 --> 00:42:36,550
And so there was some speculation that maybe this is related to the observation.

400
00:42:38,200 --> 00:42:44,800
And of course, this is complicated to calculate because there are lots of different processes to include in the calculation.

401
00:42:45,220 --> 00:42:49,390
But we have done some simulations. So this is simulation by congruence.

402
00:42:50,080 --> 00:42:59,379
So we're simulating an annulus of gas. Just the the part of this this supermassive black hole of gas is where you have

403
00:42:59,380 --> 00:43:04,720
two stellar mass black holes embedded in the gas disk and and see what happens.

404
00:43:05,260 --> 00:43:12,340
And so the so the black holes get closer and as they orbit and different radiate different velocities.

405
00:43:12,790 --> 00:43:16,270
And we're just now zooming in to see to see what happens.

406
00:43:16,340 --> 00:43:19,690
Let me just. Sorry. Okay.

407
00:43:19,690 --> 00:43:24,370
Sorry. I can advance it. So the black hole is just come close.

408
00:43:24,760 --> 00:43:28,440
And eventually they form tight binaries. And.

409
00:43:28,860 --> 00:43:32,230
And you can see that perhaps they. They've emerged.

410
00:43:32,530 --> 00:43:36,010
You could not run these simulations all the way to merger, but.

411
00:43:36,850 --> 00:43:43,180
But it's very promising that this is these. In these. In this case, you can form binaries just by single single encounters.

412
00:43:43,600 --> 00:43:48,940
You don't need the third object to form a binary. So it's much more likely to form binaries in these cases.

413
00:43:49,990 --> 00:43:58,060
And so if you zoom in, you see that we have it changing the, the colours, the colours to show it more clearly what happens,

414
00:43:58,480 --> 00:44:05,920
you know, these dense spiral streams and, and the black holes combine and get stuck in a binary.

415
00:44:06,340 --> 00:44:13,630
If you, if you neglect the gas effects you run the same simulations without gas then the objects are just scattered away from each other.

416
00:44:13,720 --> 00:44:18,610
So gas is helping them to form a binary. And here's another student, student Henry Whitehead,

417
00:44:18,640 --> 00:44:25,600
using a different kind of simulation where you actually zoom in and now you take into account the thermodynamic effects,

418
00:44:25,600 --> 00:44:29,260
which are it must make the calculations much more complicated.

419
00:44:29,470 --> 00:44:30,400
And this is what we get.

420
00:44:30,580 --> 00:44:42,700
So it's really, really awesome that you you get basically merging black holes and in this case, you produce these shock shockwaves in the gas disk.

421
00:44:42,730 --> 00:44:48,910
It's just potentially observable. If you look in, if you search for these types of signals.

422
00:44:50,350 --> 00:44:59,350
Okay. So I think I'm out of time. So let me just summarise. So you you have now something like 90 black or black hole mergers.

423
00:44:59,980 --> 00:45:07,990
These were detected by like one year ago and there are many astrophysical pathways, but there are some problems with all of these batteries.

424
00:45:07,990 --> 00:45:09,730
So now it's a completely new field.

425
00:45:10,000 --> 00:45:17,170
It's going to be interesting to see if someone can come along and suggest a completely different avenue to merge these objects,

426
00:45:17,170 --> 00:45:18,550
and maybe that will be the correct one.

427
00:45:19,180 --> 00:45:26,680
But gravitational waves can already probe astrophysical systems in new ways so we can detect globular clusters.

428
00:45:27,520 --> 00:45:33,940
And we can we cannot see globular clusters in electromagnetic bands because they are faint, only the closest ones.

429
00:45:34,210 --> 00:45:39,130
But these ones you can see all the way to the edge of the universe, basically the observable universe.

430
00:45:39,640 --> 00:45:45,290
And so that's very exciting and new type of observation that is possible with these types of instruments.

431
00:45:45,350 --> 00:45:52,240
Also, you can detect active galactic nuclei, new ways and and processes in them.

432
00:45:52,780 --> 00:45:58,840
And so there's a very bright future for gravitational wave astronomy because there are new instruments coming along.

433
00:45:59,290 --> 00:46:06,370
And and so I think it's going to be really exciting for the next decade or so and beyond.

434
00:46:06,520 --> 00:46:07,420
So thank you very much.