Congratulations to the newly minted Doctor Natacha (Naty) Altamirano on successfully defending her PhD, entitled:
“The quantum and the gravity: Newtonian and Cosmological applications”
In her thesis, Natacha looked into an innovative idea about the nature of gravity, where it is speculated that gravity is inherently a classical and not a quantum interaction. She further studied the laboratory tests of this idea, as well is its potential cosmological applications. While most of this work was with Natacha’s co-supervisor Robb Mann (and other collaborators), she also worked with me on various topics in cosmology, including holographic big bang, modified gravity, and Sunyaev-Zel’dovich (SZ) signal from galaxies (yet to appear).
Below are pictures of Natacha’s proud supervisors, as well as a celebratory cake, featuring some of her PhD work. The latter includes the only published picture of her SZ work, showing a mysterious SZ deficit for small galaxies! 😕
You can read more of Natacha’s broad, innovative and exciting research on arXiv. We wish her all the best in her future adventures 👏 🙂
First, let me start by congratulating Dr. Elizabeth (Beth) Gould on successfully defending her PhD thesis “New Views on the Cosmological Big Bang”, last September.
Beth’s graduation cake, highlighting pictures from her PhD thesis (photo credit: Joel Gould)
“Every year, top graduate students from the Faculty of Science are nominated for the W.B. Pearson Medal, which is given to a Doctoral student from each department in recognition of their creative research …
The W.B. Pearson Medal in Physics & Astronomy has been awarded to Elizabeth Gould for her research on “New Views on the Cosmological Big Bang”, with Niayesh Afshordi.”
So, please join me in congratulating Dr. Gould on successfully finishing her PhD, starting a prestigious postdoctoral fellowship, and being recognized for her creativity by the W.B Pearson Medal.
In a post last year, I talked about our search for “echoes from the abyss” with Jahed Abedi and Hannah Dykaar in the Advanced LIGO gravitational wave observations, which are smoking guns for Planck-scale structure near black hole event horizons. Such structures are not expected in classical General Relativity, but may be motivated by various versions of the black hole information paradox such as the fuzzball models of black holes in string theory, or the infamous firewall paradox. Most surprisingly, we found that the evidence seen for (a toy model of) echoes in first observational run of LIGO data can only be seen in <1% of random noise realizations.
And then, there was the response from the community. While the theorists were beside themselves with excitement (our two papers are cited close to a hundred times in 16 months), we got a long silence from observers/experimentalists with one exception. A group of of LIGO collaboration members in Albert Einstein Institute (AEI) in Hanover were quick to express (a very healthy and deserved) scepticism. We responded to their comments in our “Holiday Edition!”, but the real question was whether our results could be reproduced by people who had more experience with LIGO data.
The AEI group finally released their analysis last December (which was later updated with a 4th black hole merger event in February), and lo and behold, they found that the evidence seen for echoes (using the same dataset and model), is only seen in 2±1% of random noise models (i.e. within 1σ of what we reported). Surprisingly though, they went on to say
“The reduced significance is entirely consistent with noise, and so we conclude that the analysis of Abedi et al. does not provide any observational evidence for the existence of Planck-scale structure at black hole horizons.” !!!
What they really are saying is that, since General Relativity has been such a successful theory for the past 100 years, we don’t really think echoes are there, and we need really strong evidence (e.g., p-value of 10-6) to be convinced otherwise. Fair enough, but that is a very subjective statement. Someone like me may argue that we have known for nearly fifty years that if you consider quantum mechanics, something funny is happening at the black hole horizons. Why is the entropy proportional to horizon area? How could information get out of the horizon of evaporating black holes? We can also explain the scale of dark energy (the infamous 10-120), by assuming a “quantum equilibrium” at the horizons of astrophysical black holes. So for me the bar is probably lower. Therefore, it is important to separate the objective statistical statements (e.g. p-value) which only depend on data, from subjective “priors” that varies from theorist to theorist.
Out of the “tentative evidence” for echoes and the resulting controversy, emerged the need for a clearer understanding of what echoes should really look like. In a recent arXiv preprint: “Black Hole Echology: The Observer’s Manual”,Qingwen Wang and I provided the most comprehensive study of echo templates, and their model dependence (and independence), for a spinning black hole.
Nominal echoes from GW150914 event, for different wall distance to the horizon
We made some surprising discoveries, e.g., that the echoes decay as a power-law ∝ time-4/3, not exponentially, as we had originally assumed, or that the signal below the superradiance frequency is insensitive to initial conditions.
These findings set the stage for the new observational evidence for echoes that was about to emerge.
Echoes Strike Back!
There is a quote that I have often heard in Physics and Astronomy gatherings, but I don’t know who it can be attributed to:
“If you have to argue about statistics, it means that you need more data.”
and indeed that will be the way to unambiguously settle the argument about the significance of black hole echoes.
First came the surprising results by Conklin, Holdom, and Ren from University of Toronto, who developed a “model-agnostic” search for echoes by cross-correlating data from the two detectors and looking for periodic signals. This was very complementary to our original search, as it assumed very little about the exact template, but looked for repeating echoes that lasted much longer. Indeed, they think they see echoes in 5 of the LIGO/Virgo events that we did not find (or look for) echoes in, with p-values 0.2%-0.8% (roughly 3σ evidence).
The grand finale came about, after I gave a talk about echoes at Yukawa Institute in Kyoto, during the CosPa 2017 meeting.
During the meeting, both Shinji Mukohyama and Lam Hui asked me whether I expected to see echoes from the binary neutron star merger GW170817, which had made headlines a couple of months earlier. I first dismissed the idea as it was a very different frequency regime from what we had for binary black holes, and given the lack of any detectable post-merger signal by LIGO/Virgo, it wasn’t even clear when a black hole remnant would form, if at all.
However, it then occurred to me that we might have an opportunity to probe a very different regime, consisting of the first few harmonics of the echo chamber. This is at too low a frequency for binary black hole mergers, but is squarely within the LIGO sensitivity band. Indeed, a simplified version what the Toronto group did, with proper inverse noise weighting, gave a huge and surprising signal for echoes at 4.2σ (or p-value of 10-5) exactly where you expected for the mass and spin of a black hole remnant of GW170817 binary neutron star merger.
cross-spectrum of the two LIGO detectors (after Wiener filtering and summing over integer multiples of the frequency). The peak at 72 Hz and 1 second is the most significant in the diagram, at 4.2σ.Same cross-power spectrum as above, but plotted at 1 second after the merger. Blue region shows the expected range for echo frequency.
So, is this just a statistical or systematic fluke? Or, could it be the beginning of the end for the black hole information paradox, as well as our first crack at the quantum gravity nut? Only time will tell.
Here is a picture of Nosiphiwo (right) with her two supervisors, me and Rafael Sorkin
Here is a picture of Nosiphiwo (right) with her two supervisors, me and Rafael Sorkin …
… and a close-up of the cake we’re holding, featuring a colorful causal set, with Nosiphiwo’s work on causal set predictions for dark energy history featured in the middle. Hopefully, I will write about it later here, but in summary, it is an amazing result that shows dark energy might have been stochastic but fluctuating (i.e. everpresent) throughout cosmic history, and this is consistent with all the cosmological observations.
We wish Nosiphiwo all the best in her grand new adventures!
Featured above, from left to right are:Don Page (Yasaman’s future postdoc advisor at Univ. of Alberta), Steve Carlip (External Examiner at Yasaman’s PhD defence), Yasman herself, myself and Rafael Sorkin (Yasman’s PhD co-supervisors), and Bill Unruh (Yasaman’s former undergraduate supervisor at Univ. of British Columbia).
To find out more about Yasaman’s impressive body of work during her PhD and MSc, ranging from Astrophysical Accretion to Entanglement Entropy and Causal Sets, check out her papers on arXiv.
Last week, I attended the conference “Cosmology and the Future of Spacetime”. It brought together physicists and philosophers to discuss foundational theoretical and observational issues about emergence of spacetime and cosmology. I talked about my Reflections on Spacetime (find slides here), with a prelude on the continuous spectrum between Science and Religion, and how we can make Quantum Gravity more scientific.
Here is a new story, with a novel twist on an old story: Let’s throw out the story!
And that’s my take on Holographic Cosmology, first developed by Paul McFadden and Kostas Skenderis, as a way to understand big bang. In short we can use the properties of a quantum field theory in 3 dimensions (without time), to understand the outcome of our 3+1 dimensional big bang.
What’s new is that, working with my PhD student Beth and other collaborators, we find observational evidence for a holographic description of our Universe by analyzing the cosmic microwave background, the afterglow of the big bag. The origin of structures in the universe is one of the deepest mysteries in modern physics, and at the heart of empirical efforts to understand the big bang. It is often believed that quantum fluctuations during an early period of accelerated expansion, or cosmic inflation, have seeded these structures but the physics and origins of inflation have remained illusive. Modern advances in quantum gravity have provided strong support for a holographic conjecture, which suggests gravitational physics within a volume contains the same information as a quantum field theory on its boundary. We apply this powerful conjecture to the early universe, rewriting the observable implications of a 4-dimensional big bang, in terms of a 3-dimensional quantum field theory. Surprisingly, we discovered that some of the simplest field theories in 3 dimensions can successfully explain (nearly) all cosmological observations of the early universe. New techniques are necessary to understand the correlations in the cosmic microwave background on angles larger than 10 degrees, which is where tantalizing hints for new physics have been seen over the past 20 years.
Niayesh Afshordi 
You must be logged in to post a comment.