Paul Joseph Steinhardt (born December 25, 1952) is an American theoretical physicist whose principal research is in cosmology and condensed matter physics. He is currently the Albert Einstein Professor in Science at Princeton University, where he is on the faculty of both the Departments of Physics and of Astrophysical Sciences.
Steinhardt is best known for his development of new theories of the origin, evolution and future of the universe. He is also well known for his exploration of a new form of matter, known as quasicrystals, which were thought to exist only as man-made materials until he co-discovered the first known natural quasicrystal in a museum sample. He subsequently led a separate team that followed up that discovery with several more examples of natural quasicrystals recovered from the wilds of the Kamchatka Peninsula in far eastern Russia. Several years later, he and collaborators reported the accidental synthesis of a previously unknown type of quasicrystal in the remnants of the first atomic bomb test on July 16, 1945, at Alamagordo, New Mexico.
He has written two popular books on these topics. Endless Universe: Beyond the Big Bang (2007), co-authored with Neil Turok, describes the early struggles in challenging the widely accepted big bang theory and the subsequent development of the bouncing or cyclic theories of the universe, which are currently being explored and tested. The Second Kind of Impossible: The Extraordinary Quest for a New Form of Matter (2019) recounts the story of quasicrystals from his invention of the concept with his then-student Dov Levine, to his expedition to far eastern Russia to recover meteorite fragments containing natural quasicrystal grains formed billions of years ago.
Education and career
Born in 1952 to Helen and Charles Steinhardt, Paul Steinhardt is the second oldest of four children. He grew up in Miami, Florida, where he attended Coral Gables Senior High School while attending classes at a local university. Steinhardt received his Bachelor of Science in physics at Caltech in 1974, and his Ph.D. in physics at Harvard University in 1978 where his advisor was Sidney Coleman. He was a Junior Fellow in the Harvard Society of Fellows from 1978 to 1981. He rose from junior faculty to Mary Amanda Wood Professor at the University of Pennsylvania between 1981 and 1998, during which he maintained a long-term association with the Thomas J. Watson Research Center, and has been on the faculty at Princeton University since the Fall of 1998. He co-founded the Princeton Center for Theoretical Science and served as its Director from 2007 to 2019.
Research Inflationary cosmology Beginning in the early 1980s, Steinhardt co-authored seminal papers that helped to lay the foundations of inflationary cosmology.
Slow-roll inflation and generation of the seeds for galaxies
In 1982, Steinhardt and Andreas Albrecht (and, independently, Andrei Linde) constructed the first inflationary models that could speed up the expansion of the universe enough to explain the observed smoothness and flatness of the universe and then "gracefully exit" to the more modest expansion observed today. The Albrecht-Steinhardt paper was the first to note the effect of Hubble friction in sustaining inflation for a sufficiently long period (the "slow-roll" effect), setting the prototype for most subsequent inflationary models.
Hubble friction played a critical role in the 1983 paper by James Bardeen, Steinhardt and Michael S. Turner who were the first to introduce a reliable, relativistically gauge invariant method to compute how quantum fluctuations during inflation might naturally generate a nearly scale-invariant spectrum of density fluctuations with a small tilt, properties later shown by observations of the cosmic microwave background to be features of our universe. The density fluctuations are seeds about which galaxies eventually form. Contemporaneous calculations by several other groups obtained similar conclusions using less rigorous methods.
Eternal inflation and the multiverse In 1982, Steinhardt presented the first example of eternal inflation. Neverending inflation was eventually shown to be a generic feature of inflationary models that leads to a multiverse, the break-up of space into an infinite multitude of patches spanning an infinite range of outcomes instead of the single smooth and flat universe, as originally hoped when first proposed.
Although some cosmologists would later come to embrace the multiverse, Steinhardt consistently expressed his concern that it utterly destroys the predictive power of the theory he helped create. Because the inflationary theory leads to a multiverse that allows for every possible outcome, Steinhardt argued, we must conclude that the inflationary theory actually predicts nothing.
In 1993, Robert Crittenden, Rick Davis, J.R. Bond, G. Efstathiou and Steinhardt performed the first calculations of the complete imprint of gravitational waves on the B-mode temperature maps and on the polarization of the microwave background radiation.
Despite his criticisms of the idea, Steinhardt's major contributions to the inflationary theory were recognized in 2002 when he shared the Dirac Prize with Alan Guth of M.I.T. and Andrei Linde of Stanford.
In 2013, Anna Ijjas, Abraham Loeb and Steinhardt added to the criticisms in a widely discussed pair of papers that the inflationary model was much less likely to explain our universe than previously thought.
According to their analysis of the Planck satellite 2013 results, the chances of obtaining a universe matching the observations after a period of inflation is less than one in a googolplex. Steinhardt and his team dubbed the result the "unlikeliness problem." The two papers also showed that Planck satellite data ruled out what had been historically accepted as the simplest inflationary models and that the remaining inflationary models require more parameters, more fine-tuning of those parameters, and more unlikely initial conditions.
In 2015, the unlikeness problem was reaffirmed and strengthened by a subsequent round of measurements reported by the Planck satellite team.
In 2018, Steinhardt, in collaboration with Prateek Agrawal, George Obieds, and Cumrun Vafa, argued that inflation may also be incompatible with string theory because inflationary models generally violate constraints (sometimes called the "swampland conjectures") on what is required for a model to be consistent with quantum gravity.
Bouncing and cyclic cosmology Motivated by what he viewed as the failures of inflationary theory, including but not limited to the multiverse, Steinhardt became a leading developer of a new class of cosmological models that replace the so-called big bang with a bounce and replace inflation with a period of slow contraction preceding the bounce. The hypothetical idea that the universe began with a bang is based on extrapolating back in time, assuming that Einstein's equations of general relativity remain valid at energies and temperatures far greater than have ever been tested.
Theorists generally agree that, if there was a big bang, then, in the instants following, quantum physics effects should have created large fluctuations in spacetime. These fluctuations would have caused space-time to curve and warp and the distribution of energy to become very uneven, all of which is inconsistent with what experimentalists observe when they study the early universe. The universe is, in fact, observed to be homogeneous. Inflation was originally invented to explain the smoothness that is observed in the universe. But it is unclear how to transition from the highly uneven conditions created after a big bang to an inflationary universe and, even if a solution could be found, the inflationary theory ultimately results in a multiverse rather than a smooth universe.
The new approach removes the bang altogether, envisioning instead a smooth transition from a previous period of slow contraction to the current period of expansion. If the contraction is slow, it smooths the entire universe, and, unlike inflation there is no multiverse. By evading the infamous cosmic singularity problem associated with a big bang, a bounce avoids quantum gravity effects that produce an unsmooth universe. A natural extension of these ideas is a never-beginning and never-ending cyclic universe in which epochs of bounce, expansion, and contraction repeat at regular intervals.
In a pivotal lecture in 2021, Steinhardt explained why it is time to jettison the big bang theory and replace the bang with a 'bounce' — a smooth transition from contraction to a dense, hot universe that continues to expand and cool.
Historical development Early models with a big crunch In 2001, Steinhardt presented the first examples of these bouncing and cyclic models, referred to as "ekpyrotic," in papers with Justin Khoury, Burt A. Ovrut and Neil Turok.
These models were based on the speculative notion suggested by string theory that the universe has extra-dimensions bounded by "branes" (where "brane" is derived from "membrane," a basic object in string theory). The fiery collision and rebound of these branes is comparable to a big crunch, a violent event that would depend sensitively on strong quantum gravity effects that are not yet established and may create tremendous curvature and warping of spacetime.
In principle, the collisions can repeat at regular intervals resulting in a cyclic universe. In 2002, Steinhardt and Turok incorporated the ekpyrotic idea into an early version of a cyclic theory of the universe.
Improved models with slow contraction and a gentle bounce Recent versions of bouncing cosmology developed by Anna Ijjas and Steinhardt introduce elements that simplify and address problems with the earlier ekpyrotic proposal. They do not require extra dimensions or branes or string theory; ordinary fields with potential energy evolving in space-time, similar to inflationary models, can be used.
Instead of a violent ekpyrosis (the collision of two branes), the smoothing and flattening of spacetime occurs through "slow contraction," a period in which space contracts very little while the Hubble radius contracts a lot. By the time the bounce is reached, the universe is "supersmoothed."
The bounce is a gentle transition that can be fully computed and maintains smoothness because it is a continuous process that occurs long before quantum gravity effects become strong. There is no cosmic singularity problem, unlike theories based on the big bang.
Universal smoothing and ultralocality To test these ideas, Anna Ijjas adapted the tools of numerical general relativity, originally invented to simulate the merger of black holes and the emission of gravitational waves, to cosmology. Together with Steinhardt and collaborators, the new tools were used to study the effectiveness of slow contraction.
The study demonstrates that slow contraction is a supersmoothing cosmological phase that homogenizes, isotropizes and flattens the universe both classically and quantum mechanically and can do so far more robustly and rapidly than had been realized in earlier studies.
Beginning from wildly unsmooth and curvy starting condition, the studies verified that slow contraction smooths virtually all of spacetime due to an effect of general relativity known as ultralocality. The ultralocal effect is specific to a contracting universe, and there is no equivalent in an expanding universe, including the case of inflation. The consequent smoothing power is an unparalleled advantage of slow contraction.
Cyclic version of bouncing cosmology