The
main theory of a cyclical universe has been the theory of an
oscillating universe. To understand this theory, you must understand
the concept of critical density. Scientists have long said that if
the density of the universe in mass-energy is less than a particular
density called the critical density, the universe will keep
expanding. But if the density of the universe is greater than this
critical density, the universe's expansion will one day slow down and
then reverse. If that were to happen, the expansion of the universe
(in which the distance between galaxies increases) would turn into a
contraction of the universe (in which the distance between galaxies
decreases). At the end of the period of contraction would be a Big
Crunch in which all of the universe ends up crunched together in a
very dense state.
According to the theory of
an oscillating universe, this Big Crunch would turn into a Big Bounce
– another Big Bang that would start the universe expanding again.
The oscillating universe theory is the idea that such cosmic phases
of expansion and contraction have continued indefinitely – perhaps
forever.
The
visual below illustrates the idea:
Although it gained a few adherents in the 1970's and the 1980's, the theory of an oscillating universe has for quite some time been what we might call a dead horse. The bullet in the head of this dead horse was the discovery during the 1990's that the expansion of the universe is accelerating. This means that the current expansion of the universe will continue forever, and there will be no Big Crunch in the future. If the current expansion of the universe will continue forever, then we cannot be living in a cyclical universe that continually goes through cycles of expansion and contraction.
But
now science writer Natalie Wolchover is trying to flog this dead horse
back into life. She has an article on the Quanta web side with the
very misleading title “How the Universe Got Its Bounce Back.” You
would think from this title that there has been some new evidence or some swing
of opinion causing the idea of a bouncing, oscillating universe to
become popular again. Nothing of the sort has happened.
Wolchover
refers to a “growing number” of cosmologists interested in the
idea that the Big Bang was a Big Bounce. This is a standard
rhetorical device of people making dubious claims that some dubious
idea is becoming popular: they usually make the claim that there is
some “growing number” of experts supporting the idea. A search of
scientific papers at the Arxiv physics paper server reveals no such
trend. In 2016 there were 10 papers (out of many thousands)
referring to a bouncing universe in their title; and in 2017 there
were 8 such papers. Hardly much of an upsurge in interest. By
comparison, during 2017 there were 659 physics papers referring to
“acceleration.”
Wolchover
also tries to create a bandwagon impression by referring to the
“bounce cosmology community.” Judging from the number of papers
published, this “community” might have trouble filling the seats
at an average high school cafeteria lunch table. Then Wolchover makes
the claim that cosmologists are “highly polarized” about whether
the Big Bang was a Big Bounce. This is not correct, and the
concept of a Big Bounce is merely the speculation of a handful of
thinkers, most of whom do not even seem to claim it is a likelihood.
Wolchover
mentions the cyclical theories of Paul Steinhardt, failing to tell us
that Steinhardt's very speculative theories (which he calls the
“ekpyrotic universe” and the “anamorphic universe”) have
attracted virtually no support, with pretty much no one (other than a few of his co-authors) out there
selling his theories other than himself. Wolchover also refers us to
a recent paper by Graham, Kaplan
and Rajendran.
When I looked up the paper
by Graham, Kaplan and Rajendran I found a paper with the catchy
title “The Born Again Universe.” But the abstract reveals that
the authors are just giving us some madcap speculation, for it
reveals they are speculating about “vorticity in compact extra
dimensions” and “an NEC violating fluid of Kaluza-Klein
excitations of the higher dimensional metric.” They're not just
imagining undiscovered extra dimensions, but a spooky
spinning vortex in
undiscovered extra dimensions, along with some kind of weird wet
wackiness (and the paper also speculates about the sci-fi concept of
wormholes). This is no more substantial than speculating about purple
giants living near the star Alpha Centauri. We should not at all mistake this type of groundless runaway
speculation for either a scientific finding or something that is worthy of our attention.
Not to be confused with actual science
In case you didn't know,
there is a breed of modern theoretical physicists that love to create
incredibly elaborate speculations, like science fiction writers
imagining in great detail some imaginary planet. Such flights of
fancy should only be published in journals of metaphysics or
mathematics or in journals with titles such as “The Journal of
Cosmological Speculations.” But we have a bad situation in which
these little physicists' daydreams (with as many imaginary details as
a science fiction story) can get published in physics journals, where
people might mistake such works of imagination for actual science.
The reason why such speculations are very rarely worthy of anyone's attention is that at any moment there are more than 1,000,000,000,000,000,000,000,000,000,000 possible states that matter, energy, time and space might take; so when some physicist speculates (with lots of specific details) about just one of these possible states (without providing evidence that such a state exists) there is only a microscopic chance that such a speculation matches reality.
The reason why such speculations are very rarely worthy of anyone's attention is that at any moment there are more than 1,000,000,000,000,000,000,000,000,000,000 possible states that matter, energy, time and space might take; so when some physicist speculates (with lots of specific details) about just one of these possible states (without providing evidence that such a state exists) there is only a microscopic chance that such a speculation matches reality.
Einstein's theory of
general relativity (which has held up extraordinarily well to
observational tests) excludes the possibility of a Big Bounce. There
could have been no such bounce if Einstein's theory of general
relativity is correct. All attempts to imagine an eternal cyclical
universe are doomed to failure. The first reason is that we live in
an accelerating universe destined to expand forever. If there is
only one cycle of endless expansion ahead of us in time, it makes no sense to believe
there were an infinite number of cycles in the past, alternating between expansion and contraction.
The second reason for
rejecting a cyclical universe has to do with entropy. The Second Law
of Thermodynamics tells us that entropy is always increasing. To end
up with a universe such as ours after 13 billion years means the
universe must have had an extraordinarily low entropy at its
beginning, which is exactly what cosmologists such as Roger Penrose have
pointed out. As discussed here, in a book Penrose
states that “the improbability of the universe conditions that we
actually seem to find ourselves in” is roughly 1 in 10 to the 10 to
the 124th power, which is a probability almost infinitely
smaller than the 1 in 10 to the 10 to the 60th power
estimate he made for the chance of a planet with all of Earth's
biology appearing suddenly from random particle collisions.
Cosmologists have never explained this low entropy, and the problem of explaining it becomes many times worse if you imagine previous cycles of the universe (and infinitely worse if you imagine an infinite number of such cycles). This is because entropy would grow very much in each cycle of the cyclical universe, and there would be no magic mechanism reverting entropy to zero during a Big Bounce. This problem has been pointed out many times to theorists of cyclical universes, who just continue to ignore it, concentrating on their madcap speculations trying to squeeze a bounce out of a universe showing no signs of being compatible with such a notion.
Cosmologists have never explained this low entropy, and the problem of explaining it becomes many times worse if you imagine previous cycles of the universe (and infinitely worse if you imagine an infinite number of such cycles). This is because entropy would grow very much in each cycle of the cyclical universe, and there would be no magic mechanism reverting entropy to zero during a Big Bounce. This problem has been pointed out many times to theorists of cyclical universes, who just continue to ignore it, concentrating on their madcap speculations trying to squeeze a bounce out of a universe showing no signs of being compatible with such a notion.
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