I talked a bit on Twitter last night about the Past Hypothesis and the low entropy of the early universe. Responses reminded me that there are still some significant misconceptions about the universe (and the state of our knowledge thereof) lurking out there. So I’ve decided to quickly list, in Tweet-length form, some true facts about cosmology that might serve as a useful corrective. I’m also putting the list on Twitter itself, and you can see comments there as well.
- The Big Bang model is simply the idea that our universe expanded and cooled from a hot, dense, earlier state. We have overwhelming evidence that it is true.
- The Big Bang event is not a point in space, but a moment in time: a singularity of infinite density and curvature. It is completely hypothetical, and probably not even strictly true. (It’s a classical prediction, ignoring quantum mechanics.)
- People sometimes also use “the Big Bang” as shorthand for “the hot, dense state approximately 14 billion years ago.” I do that all the time. That’s fine, as long as it’s clear what you’re referring to.
- The Big Bang might have been the beginning of the universe. Or it might not have been; there could have been space and time before the Big Bang. We don’t really know.
- Even if the BB was the beginning, the universe didn’t “pop into existence.” You can’t “pop” before time itself exists. It’s better to simply say “the Big Bang was the first moment of time.” (If it was, which we don’t know for sure.)
- The Borde-Guth-Vilenkin theorem says that, under some assumptions, spacetime had a singularity in the past. But it only refers to classical spacetime, so says nothing definitive about the real world.
- The universe did not come into existence “because the quantum vacuum is unstable.” It’s not clear that this particular “Why?” question has any answer, but that’s not it.
- If the universe did have an earliest moment, it doesn’t violate conservation of energy. When you take gravity into account, the total energy of any closed universe is exactly zero.
- The energy of non-gravitational “stuff” (particles, fields, etc.) is not conserved as the universe expands. You can try to balance the books by including gravity, but it’s not straightforward.
- The universe isn’t expanding “into” anything, as far as we know. General relativity describes the intrinsic geometry of spacetime, which can get bigger without anything outside.
- Inflation, the idea that the universe underwent super-accelerated expansion at early times, may or may not be correct; we don’t know. I’d give it a 50% chance, lower than many cosmologists but higher than some.
- The early universe had a low entropy. It looks like a thermal gas, but that’s only high-entropy if we ignore gravity. A truly high-entropy Big Bang would have been extremely lumpy, not smooth.
- Dark matter exists. Anisotropies in the cosmic microwave background establish beyond reasonable doubt the existence of a gravitational pull in a direction other than where ordinary matter is located.
- We haven’t directly detected dark matter yet, but most of our efforts have been focused on Weakly Interacting Massive Particles. There are many other candidates we don’t yet have the technology to look for. Patience.
- Dark energy may not exist; it’s conceivable that the acceleration of the universe is caused by modified gravity instead. But the dark-energy idea is simpler and a more natural fit to the data.
- Dark energy is not a new force; it’s a new substance. The force causing the universe to accelerate is gravity.
- We have a perfectly good, and likely correct, idea of what dark energy might be: vacuum energy, a.k.a. the cosmological constant. An energy inherent in space itself. But we’re not sure.
- We don’t know why the vacuum energy is much smaller than naive estimates would predict. That’s a real puzzle.
- Neither dark matter nor dark energy are anything like the nineteenth-century idea of the aether.
Feel free to leave suggestions for more misconceptions. If they’re ones that I think many people actually have, I might add them to the list.
I’m still baffled by point 16 saying gravity is driving the expansion. In Dr. Carroll’s reply he says :
“But vacuum energy has negative pressure, equal in size but opposite in sign to its energy.”
But aren’t gravity and vacuum energy two distinct phenomena? I understand that during inflation gravity was very briefly repulsive and that’s what drove the equally brief period of inflation but that since inflation ended, gravity became attractive.
In all the popular books I’ve read (that’s the limit of my understanding) gravity has ALWAYS (excepting inflation) been attractive! Is there some deep and arcane reason that vacuum energy and gravity are somehow related. Or, to put it crudely, two sides of the same coin?
Elizabeth, you are asking what caused the Planck epoch to transition into the grand unified epoch, which is unknowable with current and anticipated instrumentation. We have no good answers for the causes of subsequent events, through the end of the inflationary epoch, after which causes become clear and mostly consistent with observations. The most that can be said on these questions is that inhomogeneities certainly arose. Whether they arose prior to or after the initiation of inflation is unknown, but they certainly existed at some point during inflation. The more interesting, or at least deeper, questions begin with what favored matter over antimatter during these times.
Sooo cool. But like, if the universe is expanding into nothing because there is nothing outside the geometry of spacetime, I don’t really understand what nothing is. How can something get bigger and expand into nothing?
I have a non technical idea from a non-physicist: i’m a composer, and I have no details. just an inkling based on the law of conservation of energy. if you determine the idea is impossible, i’ll drop it.
i’m guessing the entire universe can be expressed as energy. so the entire universe was a point smaller than a planck unit as ultra short wavelength gamma rays. being inherently unstable it didn’t much for it to lose stability and start expanding. as it expanded it cooled until we have what we have today.
Update : I’ve just read Dr. Carroll’s Dark Energy faq blog from 2011 where he explains the relationship between vacuum energy and the cosmological constant – turns out they are exactly the same thing.
So gravity is driving expansion…..who’d a thunk it?!?
One of the most helpful, informative lists of facts I have ever read. Thank you!
Vacuum energy is some kind of infinite energy T=0K
Gravity is some kind of local matter.
We don’t have theory of their relations.
===
Michelle Esther
So called ”nothing” is an infinite vacuum that is packed
with ”negative virtual particles” that somehow can appear
as real: Casimir effect, Lamb shift, vacuum’s fluctuation
====
Frank says:
. . . vacuum energy and the cosmological constant –
— turns out they are exactly the same thing.
#
The cosmological constant, the critical density in the whole Universe
is so small ( 9.9 x 10^-30 g/cm^3) that its masses cannot ”close”
the Universe into sphere and therefore the Universe
as whole is ”open”- flat.
And because the Universe (as whole) is homogeneous and isotropic
therefore its density ( 9.9 x 10^-30 g/cm^3 ) must always remain
constant. It means that the Universe (as whole) is infinite flat and cold.
(after every cosmic flat homogeneous and isotropic horizon there is
other one . . . . and so, and so, and so. . . )
===
Great post, with lots of info in one place. You say, “Roughly speaking, the “source of gravity” is the energy density of a fluid plus three times the pressure of that fluid.” I can’t find this in your book. Could you give a page or formula reference for it. Thanks.
The Conundrum of Gravity: I have a haunting sense that Gravity and the Strong Nuclear Force are cut from the same cloth (topologically flip sides of a “force” boundary encountered in nuclear particles). The gravity face of the nuclear particle projects outward indefinitely to be diluted, while the Strong Nuclear Force is encapsulated within the nucleus to enforce its’ boundary. (As gravity’s force lessens, the Universe’s expansion accelerates.) While there is another less compelling ghost haunting me that perceives gravity as a by-product of the curvature of space-time attributed to the presence of mass. However, it is not a simple issue of space-time’s displacement. Mass needs space-time to exist… subatomic spin is essential… and spin can reduce the passage of time the near standing still. In so many words, I’ve fallen hopelessly into the void of unknowns.
Dear Sean Carroll,
I see a tension between your statements 8 and 9 concerning the total energy of the universe.
If the law of conservation of energy can’t be applied to the entire universe due to Noether’s theorem, how can we balance the total energy of the universe then to zero. Shouldn’t we instead conclude, that “there is no unique value we can ever attribute to something called the energy of the universe” as done by Tamara Davis in this article in Scientific American, whether we take gravity into account or not?
https://people.smp.uq.edu.au/TamaraDavis/papers/SciAm_Energy.pdf
Surely the statement “gravity always attracts”, is not the whole truth about the universe because then nothing would make sense.
But if we mean repulsive gravity, it all gets pretty complicated because …
However, statement “to push things apart” – this has completely depraved scientific reasoning.
If you want to know more about it, invite to Twitter (@s7hummel).
Hope that my tragic English will not be a good excuse!?
So maybe a little puzzle: (terrible doubts of the repulsive gravity: which way to push the nearest superclusters of galaxies? Towards us or in the opposite direction).
Unfortunately, it is not a good answer that both options are correct.
Shouldn’t there be a certain amount of caution in assuming there is a one to one correspondence between the math and a physical basis, given that epicycles made the same assumption.
They were an excellent modeling of the universe, from our point of view and could always be added and revised, as more observations were made, because we are at the center of our point of view. The real error was assuming the one to one physical correspondence with crystalline spheres, as the underlaying basis. It certainly seems Big Bang Theory has followed a similar process, with Inflation and Dark Energy, especially. How can it ever be falsified, if enormous, otherwise unexplained forces of nature can be added, to fill any gaps that arise?
The assumption is that with so many people working on the issue, any possible errors would quickly come to light, but that overlooks the very real sociological factors, where the larger feedback loops only serve to re-enforce the prevailing wisdom. Look to other areas of human endeavor; politics, religion, economics, etc, where going against the grain takes real courage.
Math is an excellent modeling tool, but we should be careful, as any tool not used well, can cause real problems.
For example, are three dimensions foundational to space, or simply a useful mapping device? To put it in context, are longitude, latitude and altitude foundational to the biosphere of this planet, or a useful mapping device? What if you use different center points and planes, wouldn’t that constitute different sets of dimensions?
Now we are off into multiverses and everyone seems to follow along, but as these theories become ever more unaccessible, the questions will only grow more incessant, as future generations of cosmologists find the field ever more governed by politics, than science. Something about funerals and revolutions comes to mind.
Hi Sean, Thanks for a great list. I’d suggest something about the fact that the red shift of distant galaxies is not due to the Doppler effect of them “speeding away from us” but rather the expansion of space itself.
Vance
Vance,
Einstein observed that space is what you measure with a ruler and the evidence of this expansion is the redshifted spectrum of intergalactic light. Yet how is it redshifted, if not because the light takes longer to cross? In many regards, it is the speed of light that is the given “ruler” of space. So which is the real ruler?
As Rob Manning observes, given the rate of expansion, the universe has grown to some 94 billion lightyears. How is it that we use two metrics of space, both based on the same intergalactic light, and not see a conflict?
When redshift was first observed, it was assumed to be a simple expansion in space, but when it was realized the rate of increasing redshift is the same in all directions, the consequence of which would mean we are at the center of the universe. So then it was changed to an expansion OF space, based on the premise of spacetime. Then every point would presumably appear as the center. Yet if the light is being redshifted, by taking longer to cross this expanding space, then it is obviously not Constant to intergalactic space, so using the physical explanation for GR would seem to overlook the premise on which spacetime is based.
To use the analogy of the inchworm crawling on an expanding balloon, both these metrics are based on the very same light. Is there some extra-dimensional explanation for that?
The first letter is “A”. Shouldn’t that be “I”?
Hate to see minor goof detract from otherwise excellent essay.
Fixed!
Re: 18 We don’t know why the vacuum energy is much smaller than naive estimates would predict. That’s a real puzzle.
What about applying the holographic principle to Hubble sphere. If the degrees of freedom within the Hubble sphere are only on the order of the area in Planck units, the vacuum energy comes out on the right order of magnitude. So this suggests we’re just over counting the degrees of freedom.
For Ms. Greenberg:
You might derive some benefit from the poem from which the motto of this blog appears to be taken, i.e. ‘atoms and the void’. This is “On the Nature of Things” by the latin poet Lucretius written about two millennia ago. It is still the best attempt to think through the implications of what Lucretius and we can see are the fundamentals of nature. Nothing in contemporary science changes the relevance of any of his conclusions about how to live in a world that is just atoms and the void. At a minimum it is better than just trying to make decisions only as an isolated individual being. Some comfort may be obtained in realizing that humans have understood all you are saying for a very long time.
Sean,
All these assertions philosophically represent fundamental ideas about the universal nature of reality and its evolution. The first six items basically frame epistemological observations about foundations for “the Big Bang.” Item seven is ontologically special as a fundamental question about ‘causality;’ especially, since it barely avoids coming close to those mythical references within popular cultural debates about “Why . . . is there something rather than nothing?” Items eight through ten generally begin to make descriptive assertions about the “laws of nature.” Items eleven through nineteen characteristically relate to observed and predictive outcomes.
I am fully convinced of the inherent existential mechanisms that give rise to the emergence and persistence of “genetic lawfulness.” In this collective context, coherency and consistency conceptually demand that ontology genetically comes before epistemology, especially in any demonstrable terms of physicality. In this very general sense of categorical limits, it is very easy to conclude that, “natural laws” must emerge out of precursory processes. The physical constraints of the phasic thresholds of emergence observationally place the universe at the culminating processes of the “Big Bang.” So, through demonstrable principles, I am very comfortable with the suggestion that the universe can exist all by itself as a self-contained system. But your argument concludes “the universe simply is, without ultimate cause or explanation.” Like the ontological argument for the existence of “God” and other suppositions which are similarly misconceived, it begs a deeper understanding and explanation if you really want to avoid the fallacies of infinite regressions. Do you have that conceptual insight? To stress the point, simple “being” does not explain the very real physics before and after the “Big Bang.”
CAMIII
1. In regard to #7:
“The universe did not come into existence “because the quantum vacuum is unstable.” It’s not clear that this particular “Why?” question has any answer, but that’s not it.”
As I understand it from his “Why is there something rather than nothing?” paper, Dr. Carroll leans towards the idea that the best answer to this question is that “something” just is. It’s a brute fact. I don’t agree. Another way of saying this question is to say that you start with 0 (“nothing”) and then end up with 1 (“something”). You can’t do this (can’t change “nothing” into “something”) unless somehow the 0 isn’t really 0 but is actually a 1 in disguise, even though it looks like 0 (“nothing”) on the surface. This idea feeds into a comment on #5, below.
2. In regard to #5:
“Even if the BB was the beginning, the universe didn’t “pop into existence.” You can’t “pop” before time itself exists. It’s better to simply say “the Big Bang was the first moment of time.” (If it was, which we don’t know for sure.)”
I think it’s possible that there can be “nothing” at the beginning of the universe. This can be viewed in two different ways. From one way of thinking, this “nothing” just looks like nothing. From another way of thinking, it looks like “something”. After the fact, the human mind can view the switching between the two different words/perspectives as a temporal change, or a “popping into existence” when in fact there was no “popping into existence”; just a change from one mental perspective to another.
Am i correct in seeing some tension between #1 (Big Bang Model near 100%) and #11 (Inflation at 50-50)?
I thought the BBM implicitly included inflation, or does the period covered by the BBM end before inflation started?
If not, then, just was you distinguished between the Big Bang “model” and “event”, is a similar distinction needed for the “inflationary period” and the “inflationary mechanism”?
In other words, what is it specifically about inflation that is 50-50, and is it in any way related to the BBM?
As to why the universe had a low entropy there are two alternatives 1) the processes that led to universe creation somehow naturally lead to a low entropy beginning or 2) only a low entropy universe can survive and lead to a universe with life ie conscious observers.
Robert Thomas,
That’s a distinctly Western point of view. More the node side of the equation, than the Eastern, network side. The whole yin/yang, reality as a tension and balance of opposing elements.
Even the concept of time is different. As we view ourselves as entities moving through the environment, we think of the future as in front and the past behind. While the Eastern view is the past is in front and the future behind, because the past is known and we see what is in front, while the future and what’s behind are unknown.
Now we do see events after they occur and the light flows onto other events, but we are also entities moving through space.
Even space is both equilibrium and infinity.
Another description of reality is Emerson’s; “We are but thickened light.”