Ishak-Boushaki Conducts Research at the Intersection of “Modern Cosmology and General Relativity”

by Haywood McNeill

The good news is physicists say the Universe is expanding.

And they say it is expanding at an accelerating rate.

The bad news is they are not sure why the acceleration is happening.

The puzzle has placed the work of 20th century genius Albert Einstein front and center. That the Universe is expanding contradicts Einstein’s initial thought – that the Universe is static or at equilibrium, neither expanding nor contracting. Einstein included a term in equations of his general theory of relativity to allow for a static universe, which was the prevailing thought at the time.

This brings us to UT Dallas physics Professor Mustapha Ishak-Boushaki, PhD, who’s conducting research “at the intersection of modern cosmology and general relativity.” He studies the large-scale Universe with an emphasis on the mechanics of how the Universe operates and evolves. 

Ishak-Boushaki and others trying to understand the Universe’s accelerating expansion are investigating whether the acceleration is caused by some unknown “dark energy” or by modifications to gravity (i.e., General Relativity of Einstein). 

Ishak-Boushaki was one of the first cosmologists to suggest that these two questions may be intertwined. In a seminal article he led and published with collaborators from Princeton in 2006, a new area of research in the field was opened.  
(link: https://journals.aps.org/prd/abstract/10.1103/PhysRevD.74.043513 ). 

Ishak-Boushaki continues to work on this question and involves his graduate students in moving the field forward. In 2018, he accepted an invitation to write a 200-page review article on the subject in the prestigious Living Reviews in Relativity, the first time a UTD faculty member has been invited to do so. 
(link: https://link.springer.com/article/10.1007%2Fs41114-018-0017-4). 

Dark energy

 “The real problem is cosmic acceleration, and one of the possible solutions is dark energy. In the last 20 years, cosmological observations indicate that cosmic acceleration, and the dark energy associated with it, are caused by some energy or dynamic component that has the properties of what is called a cosmological constant. This is an additional term that comes in Einstein’s equations. 

“But we note that the measurements indicate that this constant term is significantly different in its numerical value from the cosmological constant introduced by Einstein and in fact does quite the opposite of what Einstein was aiming for: it does not make the Universe static but rather causes its expansion to accelerate!  

“It is an energy component of the universe that instead of having an attractive gravity, has repulsive gravity. Dark energy has negative pressure. And if you take that and you insert it in the gravitation equations of General Relativity, it can make the Universe accelerate,” Ishak-Boushaki said.

Repulsive gravity means that galaxies are not only receding from each other, but they are doing it as if some force is pushing them apart and causing them to accelerate away from each other. 

Ishak-Boushaki added that the cosmological constant could be like a simple constant of nature governing gravitation. But there are issues with this possibility.

“Indeed, if we try to match these equations with quantum physics, then this cosmological constant is indistinguishable from quantum vacuum energy that is many orders of magnitude larger according to quantum field estimations,” Ishak-Boushaki said.

Quantum vacuum energy is an underlying background energy that exists in space throughout the Universe.

“Before the discovery of cosmic acceleration in 1998, the expectation of cosmologists was that the expansion of the Universe will be either at a constant rate or slowing down.”

Ishak-Boushaki said it is important to stress that mathematically, the cosmological constant is fine and can be as regarded as Newton’s constant – both can be measured just from the observations. However, he said, the cosmological constant is more profound, and the currently measured value is not consistent at all with some estimations from quantum field theory that indicate such a cosmological constant must be orders of magnitude larger. 

“Today we know that dark energy is extremely close to a cosmological constant, but we do not know if there is necessarily any physical component associated with it. But, if so, then how can it be reconciled with the huge quantum field theory expectations?” Ishak-Boushaki asked. 

The pull of UT Dallas

When he arrived in 2005, Ishak-Boushaki said that he liked the direction the University is going, and that direction played a role in his coming here.

“The impression I had when I first visited UTD is I felt that the floor is going up. It is a university that is ascending. I felt I can create a large group, expand and flourish. It’s not the kind of university that’s on a horizontal path or where you are given a square to fit in. I think that was an encouraging point to join UTD. And this proved to be 100% true!” Ishak-Boushaki said.

He has taught the graduate physics courses of Special Relativity and General Relativity, Cosmology, Classical Mechanics as well as two different undergraduate courses of Mechanics. His unconventional teaching methods earned him in 2008 and 2018, the Award for Outstanding Teacher of the Year at the School of Natural Sciences and Mathematics, and the 2021 President’s Graduate Teaching Excellence Award at UT Dallas. He also received the 2013 Robert S. Hyer Award for Excellence in Research and Mentoring from the Texas Section of the American Physical Society. And most recently, he has been elected in 2021 as Fellow of the American Association for the Advancement of Sciences (AAAS), one of the most prestigious recognition of U.S. scientists.  

“My graduate courses range in general from 10 to 20 students while my undergraduate large classes are near 300 students. I mentored 10 PhD students to graduate in the last 16 years, and I am very proud about where they are and what they have accomplished. For example, Michael Troxel is a faculty member at Duke; Eske Pedersen is a researcher at Harvard; Austin Peel is at École polytechnique fédérale de Lausanne, or EPFL (Swiss Federal Institute of Technology at Lausanne); and Weikang Lin and Ji Yao are at Shanghai University,” Ishak-Boushaki said.

“Some of them (Dossett, Troxel, Weikang) received the best dissertations awards at UTD, and the President’s Award for best dissertation. I have 5 graduate students now, and we are very excited about our research projects. 

“In sum, my efforts with my students and our accomplishments these last 16 years have made us one of the leading groups on the question of cosmic acceleration and testing general relativity at cosmological scales. I look forward to the next few years as we start to receive the precise quality data to get to the bottom of some of these questions. I am very excited about that!” he said. 

Closing in on the answer

Ishak-Boushaki described the cosmological acceleration as a science blossoming in many directions: “It’s related to cosmology and the Universe, to particle physics via the vacuum energy and the cosmological constant; gravitation, because of general relativity; and it’s related to unifying theories of physics like string theory and other mathematical theories that try to unify physics.”

“The Acceleration of the Universe and the Dark Energy associated with it are one of the most challenging and most important problems in physics. The National Aeronautics and Space Administration (NASA), the National Science Foundation (NSF), and the United States Department of Energy (DOE) all are spending large sums of funding to try to understand this problem,” Ishak-Boushaki said.

“In order to explore the problem of cosmic acceleration, NASA has put out its space telescope, the Nancy Grace Roman Space Telescope (formerly known as WFIRST, the Wide Field Infrared Survey Telescope). NSF and DOE are sponsoring the Legacy Survey of Space and Time (LSST), and DOE is sponsoring the Dark Energy Spectroscopic Instrument (DESI).

“I am heavily invested, along with my graduate students, in leading roles in LSST and DESI survey collaborations that will revolutionize our understanding of cosmic acceleration and the nature of gravity at cosmic scales. LSST-DESC in 2020 gave me builder status – there are only 35 builders out of 1100 members, so this is quite an honor. 

“The efforts I am currently putting into DESI and then LSST surveys will allow us to get to the bottom of this and provide decisive answers to this question. As it is now with the current available data from other surveys, General Relativity of Einstein still is consistent with observations, but some small tensions are present to leave some small suspense whether this still will be the case when we will analyze the data from DESI in 2023 and LSST-DESC a couple of years after that. We are thrilled for the opportunity!” he said.