Prof of Old School

[Intro]
High in the halls of the grads who are gone
The prof would write with their ghosts
The ones he had lost and the ones he had found
And the ones who had loved him the most

[Verse 1]
The ones who’d been gone for so very long
He couldn’t remember their names
They spun him around on the damp old school
Spun away all his sorrow and pain

[Chorus]
And he never wanted to leave, never wanted to leave
Never wanted to leave, never wanted to leave

[Verse 2]
They wrote through the day
And into the night through the snow that swept through the hall
From winter to summer and winter again
‘Til the walls did crumble and fall

[Chorus]
And he never wanted to leave, never wanted to leave
Never wanted to leave, never wanted to leave
And he never wanted to leave, never wanted to leave
Never wanted to leave, never wanted to leave

[Outro]
High in the halls of the grads who are gone
The prof would write with their ghosts
The ones he had lost and the ones he had found
And the ones who had loved him the most

#JennyofOldstones

Farewell to Dr Chiamaka Okoli

Chiamaka’s PhD defence on Dec. 4th, 2018 (image courtesy of Natacha Altamirano)

It is with great sadness that I share here the news of passing of my former PhD student, Chiamaka Okoli. Chiamaka successfully defended her PhD with the title “Dark Matter and Neutrinos in the Foggy universe” last December. Her PhD convocation at the University of Waterloo was scheduled for last week, on June 13th.

Post-defence celebration with cake at PI bistro; From left to right: James Taylor (co-supervisor), Chiamaka, Larry Widrow (Chiamaka’s PhD external examiner), me and my son Juyah, and Mike Hudson (images courtesy of Natacha Altamirano)

However, that was not meant to be. Chiamaka’s life was cut short on June 6th in McMaster hospital in Hamilton. You can read more about Chiamaka on Perimeter Institute’s website. If you wish to contribute to The Chiamaka Okoli Trust Fund, to help her family with funeral costs (which involves moving her body back to her native Nigeria), please contact Jamie Cooper as soon as possible.

But then, here is my story …

I first met Chiamaka in Fall 2012, when she joined the then recently established Perimeter Scholar International program. She had just finished the diploma program with Ravi Sheth at the International Centre for Theoretical Physics in Trieste, and was eager to work more on cosmological structure formation. We started working on understanding the profiles of dark matter haloes, establishing a novel paradigm to predict their concentration based on energy conservation. This led to Chiamaka’s first paper.

Figure 5 from Chiamaka’s first paper, comparing her prediction for concentration of dark matter haloes as a function their mass, with previous work.

Chiamaka then started her PhD program at the University of Waterloo, working with me and James Taylor.

Chiamaka’s second paper studied a novel possibility for tracing the fingerprints of cosmic neutrinos by how they could slow down the motion of dark matter haloes through dynamical friction. We predicted that, with proper modeling, this effect could be detected in current and future galaxy surveys.

A figure from Chiamaka’s second paper , showing how wakes of neutrinos can slow down clustered dark matter haloes, through their gravitational pull

And there was much more:

  • In her third and final publication, Chiamaka established the range of theoretical uncertainties in predictions for annihilation signal from dark matter haloes
  • She was working with me and Ue-Li Pen to test her theoretical predictions for dynamical friction due to neutrinos using TianNu simulations
  • She was working with Natacha Altamirano, Utkarsh Giri, and I to measure and understand the thermal Sunyaev-Zel’dovich effect from groups of galaxies, as seen by Planck satellite

So long, farewell …

Chiamaka’s final two years on Earth were embodiments of perseverance in the face of adversity. In August 2017, Chiamaka gave birth to her baby boy, Munachi. The same month, her mother passed away back home in Nigeria. In November 2017, while still on maternity leave, she submitted two papers to arXiv, and then started applying for postdoc positions. In January 2018, she came back to work. In February 2018, she was hit with her first near-fatal cerebral aneurysm, which she talked about in a facebook post, in its one-year anniversary. It took a few surgeries, as well as weeks and months of recovery in the McMaster hospital, as well as Grand River hospital and Rehab center for her to get back on her feet. Meanwhile, she was being contacted for postdoc interviews and offers. In September 2018, Chiamaka again came back to work, determined to wrap up her PhD thesis, which she managed to do by mid-October. She defended her PhD in December 2018. In March 2019, Chiamaka had a final cranioplasty surgery in Hamilton, so that she’d fully recover from her first aneurysm episode. Alas, it struck again in May, and ended her life.

Even though Chiamaka had postdoc offers, she ended up turning them down, and instead started looking for data science jobs in the area. The exact reason remains a mystery to me, but I would like to think the ordeal that she had gone through had given her a new perspective on life. I also would like to think that I understood all her struggles, and helped her as best as I could, even though that might just be wishful thinking.

Like all academics, Chiamaka’s legacy now propagates through those who read and study her work. They are all those open-ended questions and ongoing projects that will permeate through journals and workshops, along with the dreams of what she could have done with them, only if universe treated her more kindly.

And finally, her legacy continues through her 22-month old son, Munachi, who is still too young to know what is happening. I do hope I get to tell him at some point about his mother’s enthusiasm for cosmology, zest, determination and sense of humor (even when she couldn’t talk and had to write her thoughts, while in a hospital bed).

Farewell Chiamaka.

Image may contain: 1 person, sitting

HC SVNT DRACONES!

I gave a talk about dragons!

Here be Dragons: Ancient cartographers often used illustrations of dragons to show treacherous and uncharted territories. Physicists, Astronomers and Cosmologists have managed to vanquish these dragons out to the farthest reaches of the cosmos, highest temperatures imaginable, and deepest holes in the galaxies. Beyond these boundaries lie our new age dragons. I will retell the tales of these creatures and our battles to slay them.

(Physics 10, University of Waterloo, September 2018; keynotePDF)

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Congratulations to Dr. Mansour Karami!

Congratulations to Dr. Mansour Karami, who has just successfully defended his PhD thesis, entitled:

“Probing the dark universe with gravitational lensing”

Mansour’s PhD focused on different ways in which gravitational lensing teaches us about dark objects in the universe:

Here is a picture of Mansour, with his proud co-supervisors, and a cake that features pictures from his thesis:

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From Left to Right: Me marveling at the cake, the freshly minted doctor, and his other proud co-supervisor, Avery Broderick (photo courtesy of Jorge Preciado)

And here is a picture of the said cake, which you can understand better by reading Mansour’s thesis!

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Mansours’ PhD cake!

We wish Mansour all the best in his future adventures in the world of quantitative finance 👏😲😉

Congratulations to Dr. Natacha Altamirano!

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! 😕 IMG_3950

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You can read more of Natacha’s broad, innovative and exciting research on arXiv. We wish her all the best in her future adventures 👏 🙂

Congratulations to Dr. Elizabeth Gould!

This news post is a bit late but for good reason!

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.

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Beth’s graduation cake, highlighting pictures from her PhD thesis (photo credit: Joel Gould)

Beth’s thesis focused on innovative ways in which physics could be different as we approach big bang. Could it be that Quantum Mechanics is “out of equilibrium”, or that spacetime really has one more or less dimension?  Or, could it be that history literally repeats itself, as our future is the same as our past (what we call periodic time cosmology)?  In fact, one of Beth’s papers was even cited by the late great Stephen Hawking, in his final publication.

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Beth, along with Auzaud and me, posing with the said cake! (photo credit: Joel Gould)

Beth is my 7th graduating PhD student. After a short postdoc at the University of Southampton this year, Beth will be joining the brand new Arthur B. McDonald Canadian Astroparticle Physics Research Institute, at Queen’s University as a postdoctoral fellow, in September 2018.

And here is that last bit of good news that came out this week:

“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.  

Echoes strike back!

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Black Hole echoes, as rendered by Kaća Bradonjić (@physartphil)

 

 

Invitation

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.

 

 

 

While this was widely covered by science journalists (see here for some links), two of my favourites were by Zeeya Merali on Nature News and Sabine Hossenfelder @skdh on Aeon.

Headline from Sabine's article about "Echoes from the Abyss"

Controversy

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 latter was a long time coming, partly because big collaborations work slowly, and partly because of other exciting discoveries (such as the first binary neutron star merger, seen by the LIGO collaboration). In the meantime, I also organized a workshop on “Quantum Black Holes in the Sky?”, along with Vitor Cardoso and Samir Mathur, where we discussed various observational and theoretical aspects of  black hole echoes, and more.

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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.” !!!

Needless, to say that this didn’t make too much sense to us, a point that I made publicly on arXiv, and on a (let’s say less diplomatic) exchange on facebook, with Thomas Dent.

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.

For more, have a look at Sabine’s update on the state of echoes and controversy in Quanta Magazine.

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James O’Brien for Quanta Magazine

Black Hole Echology

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.

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Nominal echoes from GW150914 event, for different wall distance to the horizon

Qingwen Wang

 

 

 

 

 

 

 

 

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. Echoes_Western_Mar_2018.001

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).

Echoes_Western_Mar_2018.001

The grand finale came about, after I gave a talk about echoes at Yukawa Institute in Kyoto, during the CosPa 2017 meeting.

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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.

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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.

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Same cross-power spectrum as above, but plotted at 1 second after the merger. Blue region shows the expected range for echo frequency.

For more details, you should read our paper, watch my talk, and/or see my presentation slides.

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.

Congratulations to Dr. Nosiphiwo Zwane!

Congratulations to Dr. Nosiphiwo Zwane on successfully defending her PhD thesis, entitled:

“Cosmological Tests of Causal Set Phenomenology”

Here is a picture of Nosiphiwo (right) with her two supervisors, me and Rafael Sorkin …

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… 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!

Congratulations to Dr. Yasaman Yazdi!

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Congratulations to Dr. Yasaman Yazdi on successfully defending her thesis, entitled: “Entanglement Entropy of Scalar Fields in Causal Set Theory”

Featured above is the most photogenic celebratory cake that you’ll ever see, featuring Yasman’s seminal recovery of entropy “area” law for 2D causal sets, as well as a tale of our foray into firewall territory with high energy neutrinos!

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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.