“In the end, the most improbable and hence the most puzzling aspect of space is its very existence. The simple fact that we live in an apparently smooth and regular three dimensional world represents one of the greatest challenges to the developing quantum theory of gravity,” wrote physicist Lee Smolin in Three Roads To Quantum Gravity. “If you look around at the world seeking mystery, you may reflect that one of the biggest mysteries is that we live in a world in which it is possible to look around, and see as far as we like. The great triumph of the quantum theory of gravity may be that it will explain to us why this is so.”
Two major mysteries about the large-scale structure of the universe may be explained by tiny quantum fluctuations in the early universe: “The primordial fluctuations we are talking about occur at the incredibly small Planck scale,” said Brajesh Gupt, formerly a postdoctoral researcher at Penn State, currently at the Texas Advanced Computing Center of the University of Texas. “A Planck length is about 20 orders of magnitude smaller than the radius of a proton. But corrections to inflation at this unimaginably small scale simultaneously explain two of the anomalies at the largest scales in the universe, in a cosmic tango of the very small and the very large.”
Pre-Exists Physical Law
Quantum fluctuations can pre-exist, that is precede physical law, according to Sten Odenwald, an astronomer at the NASA Goddard Spaceflight Center, “So in the Big Bang, the establishment of ‘law’ came after the event itself, but of course even the concept of time and causality may not have been quite the same back then as they are now.”
The Penn State study, which appears online July 29 in the journal Physical Review Letters, also provides new predictions about the universe that future satellite missions could test.
Tiny Inhomogeneities in the Cosmic Microwave Background (CMB)
“While a zoomed-out picture of the universe looks fairly uniform,” reports Penn State, “it does have a large-scale structure, for example because galaxies and dark matter are not uniformly distributed throughout the universe. The origin of this structure has been traced back to the tiny inhomogeneities observed in the Cosmic Microwave Background (CMB)—radiation that was emitted when the universe was 380 thousand years young that we can still see today. But the CMB itself has three puzzling features that are considered anomalies because they are difficult to explain using known physics.”
“While seeing one of these anomalies may not be that statistically remarkable, seeing two or more together suggests we live in an exceptional universe,” said Donghui Jeong, associate professor of astronomy and astrophysics at Penn State and an author of the paper. “A recent study in the journal Nature Astronomy proposed an explanation for one of these anomalies that raised so many additional concerns, they flagged a ‘possible crisis in cosmology.’ Using quantum loop cosmology, however, we have resolved two of these anomalies naturally, avoiding that potential crisis.”
Research over the last three decades has greatly improved our understanding of the early universe, including how the inhomogeneities in the CMB were produced in the first place. These inhomogeneities are a result of inevitable quantum fluctuations in the early universe. During a highly accelerated phase of expansion at very early times—known as inflation—these primordial, miniscule fluctuations were stretched under gravity’s influence and seeded the observed inhomogeneities in the CMB.
Loop Quantum Cosmology
“To understand how primordial seeds arose, we need a closer look at the early universe, where Einstein’s theory of general relativity breaks down,” said Abhay Ashtekar, Evan Pugh Professor of Physics, holder of the Eberly Family Chair in Physics, and director of the Penn State Institute for Gravitation and the Cosmos. “The standard inflationary paradigm based on general relativity treats space time as a smooth continuum. Consider a shirt that appears like a two-dimensional surface, but on closer inspection you can see that it is woven by densely packed one-dimensional threads. In this way, the fabric of space time is really woven by quantum threads. In accounting for these threads, loop quantum cosmology allows us to go beyond the continuum described by general relativity where Einstein’s physics breaks down—for example beyond the Big Bang.”
The researchers’ previous investigation into the early universe replaced the idea of a Big Bang singularity, where the universe emerged from nothing, with the Big Bounce, where the current expanding universe emerged from a super-compressed mass that was created when the universe contracted in its preceding phase. They found that all of the large-scale structures of the universe accounted for by general relativity are equally explained by inflation after this Big Bounce using equations of loop quantum cosmology.
In the new study, the researchers determined that inflation under loop quantum cosmology also resolves two of the major anomalies that appear under general relativity. The researchers also produced new predictions about a fundamental cosmological parameter and primordial gravitational waves that could be tested during future satellite missions, including LiteBird and Cosmic Origins Explorer, which will continue improve our understanding of the early universe.
Source: Abhay Ashtekar et al, Alleviating the Tension in the Cosmic Microwave Background using Planck-Scale Physics, Physical Review Letters (2020). DOI: 10.1103/PhysRevLett.125.051302
The Daily Galaxy, curated and edited by Max Goldberg, via Gail McCormick, Pennsylvania State University