Phillip James Edwin Peebles is a Canadian-American astrophysicist, astronomer, and theoretical cosmologist who is currently the Albert Einstein Professor of Science, Emeritus, at Princeton University. He is widely regarded as one of the world's leading theoretical cosmologists since the 1970s, with major theoretical contributions to primordial nucleosynthesis, dark matter, the cosmic microwave background, and structure formation.
Professor Jim Peebles was awarded half of the Nobel Prize in Physics in 2019 for his theoretical discoveries in physical cosmology.
In an exclusive e-mail interview with The Business Standard, the distinguished Nobel Laureate Professor Phillip James Edwin Peebles talked about the origin of the universe and other aspects of cosmology.
The Business Standard (TBS): What does the evidence tell us about the origin of the Universe and what are your personal views of the evidence?
Phillip James Edwin Peebles (PJEP): We have persuasive evidence that our universe expanded from a hot dense early state, largely from careful examination of two fossils from the early state, a sea of thermal radiation at measured a present temperature of 2.725 Kelvin, measured from the absolute zero of thermodynamics, and the abundances of helium and deuterium, the stable heavy isotope of hydrogen. The properties of the radiation have been examined in great detail, by the Planck satellite and other observatories, and match up to the predictions of this hot big bang theory in remarkable detail.
Incidentally, the name big bang is misleading because it connotes an event in space time. The theory describes the close to homogenous evolution of the universe from very large densities and temperatures. But the name seems fixed so I use it.
And I might add that there are ideas about what the universe was doing before it was expanding, as inflation, but we have little in the way of tests.
TBS: The Universe is 13.8 billion years old, but it has expanded to 46 billion light-years. Could you explain how the Universe got so big? Doesn't faster than light expansion during the cosmic inflation violate Relativity?
PJEP: As in special relativity, local physics limits relative speeds to that of light. But Einstein's general theory of relativity allows motion away from us faster than the speed of light; we just can't see such objects until they slow down by the attraction of gravity.
TBS: What holds the Universe together?
PJEP: Nothing, the evidence is the universe is flying apart at greater than escape speed.
TBS: If everything expands, how can we measure the expansion of the universe?
PJEP: You and I are not expanding, and the galaxy we live in is not expanding, apart from astrophysical process that produce winds. But the distances between the galaxies are increasing. This is on average, apart from motions of galaxies with concentrations massive enough to have broken away from the general expansion
TBS: Can light be contained and how do we observe light from the Big Bang?
PJEP: We observe light, as microwaves, that are remnant from the early stages of expansion. When the expanding universe had cooled to about 3000 Kelvin the primeval plasma cooled enough to combine to neutral atoms and molecules. Free electrons in the plasma scattered the radiation and kept it from moving very far. After combination the radiation moved almost freely. But the radiation was there before that.
TBS: What causes the Cosmic Microwave Background (CMB) to vibrate exclusively in the microwave frequency range? Why not some other frequency range, or at a variety of frequencies? What does this tell us about the nature of space or the conditions at the time of the big bang, or both?
PJEP: The radiation has a thermal spectrum; remember Planck's blackbody radiation. It cooled as the universe expanded, now to a temperature at which the radiation is present at microwave wavelengths. It was much hotter much earlier.
TBS: Before recombination at 380,000 years after big bang the universe was opaque to radiation due to Compton scattering. Then why isn't it opaque today after the reionisation that occurred between 500 million and 1 billion years ago?
PJEP: The density of matter was a lot smaller at reionisation at relatively late times than at recombination in the early universe. The much lower density implies much less scattering. The effect of scattering after reionisation is thought to be observed, but it's small.
TBS: In the early universe, during inflation, would not all matter/energy be uniformly distributed and uniformly expanding? If not, why not and at what point during inflation would it no longer be uniform?
PJEP: Yes, it would have been very close to uniform but disturbed slightly by the quantum fluctuations that were frozen in by the rapid expansion.
TBS: The 2nd law of thermodynamics states that Entropy i.e. the disorder in the Universe, must increase with time. Does this mean that a closed Universe will violate the 2nd law of thermodynamics?
PJEP: No, the increase of entropy means the universe at a given baryon mass density would be hotter as it is cooling thanks it was expanding. The problem with the accumulation of entropy is in an oscillating universe.
TBS: Can we compare the early Universe to a black hole?
PJEP: Yes, people have remarked that the expanding universe is somewhat the time reversal of a collapse to a black hole. Sort of. I can't get too excited about this.