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.
Sadovnik,
My point is that black holes do function as a vortex and as that article observes, we don’t fully understand the ones on the surface of the planet.
Given there are enormous jets of energy shooting out the poles of galaxies/black holes, that increase when large masses fall in, it would seem there is some cyclical dynamic at work, just as with convection cycles here on this planet, so that proposing this “hole” leads into some abyss, rather than being one side of the cycle, seems to overlook the obvious.
Our measure of space, based on mass, contracts into galaxies, while our measure of space, based on radiation, expands between them. Could it be that what Hubble discovered was actually evidence of Einstein’s original Cosmological Constant? The balance to gravity.
We keep having to add patches to the Big Bang Theory, every time observations don’t meet predictions. Logic and history would suggest we have overlooked something. Cosmology overlooks the premise of falsifiability to its own peril.
Sadnvik,
An analogy for ignoring falsification; What if an accountant found a discrepancy in the books?
Do they write in a number to fill the space and call it, “dark money?” Leaving others to figure out what dark money is. Or do they go back and take the books apart, until they find the problem, from a misplaced decimal point, to fraud.
What we have today is not really cosmology, but more specifically, Big Bangology, because no one could even conceive of getting a job in the field, unless they basically swear fealty to the notion the universe popped into existence, 13.8 billion years ago. So any new discoveries have to be shoehorned into this model and everyone will go along, searching for the dark money. Should anyone propose anything different, they will be outcast.
At some point in the future, the sociologists will have a field day with this.
Sadovnik,
I’m sure there are far more than that who work under the assumption that Dark Energy is real, but the fact remains the only evidence is the difference between a prediction and subsequent observations.
In other science fields, that is usually a case for falsification, or the consideration of it, yet there wasn’t even a flicker of doubt when this discrepancy was first announced and Dark Energy was proposed.
Why? Can you explain why there were no questions raised? Is that science, or belief?
Inflation, Dark Matter and now Multiverses are also enormous patches or projections(aka, reductio ad absurdum) of this model.
Cosmology went through something similar, with epicycles. The math was brilliant, in that it could predict alignments well into the future, but the actual physics, for this perception of cosmic order, was wrong. How far will this current model be pushed, before there is serious review?
If I may make a prediction, it will be that the James Webb space telescope finds evidence of ever further galaxies in the cosmic background radiation, as it is my opinion it is the solution to Olber’s paradox. The light of infinite sources, shifted off the visible spectrum.
Dear Prof Carroll,
If as you say in a prior blog that “[Dark energy] does not have a frame of rest (as far as we can tell); that dark energy looks the same no matter how you move through it”, how does it answer to Mach’s Principle and does it potentially aid Sciama’s (1960s) efforts to describe the inertia of matter in terms of the expanding shells of the most distant receding stars. Anyway, do we not consider the CMBR or the Cosmic Neutrino Background as the inertial frame with respect which we note the Universe’s (absolute no less) acceleration? If the General Theory fails to properly embrace Mach’s principle because of its viable vaccuum solutions then does Dark energy help or hinder?
How is the Cosmologcial constant different/ the same as the Gauge potential, A of semi classical electrodynamics? If it is a free parameter that is allowed because it does not mess up the field equations is it any less of a mathematical artifact than the gradient of a scalar field is that gives rise to the gauge freedom in A? If as Aranhov and Bohm (AB) experiment tells us that A is actually of ontological interest (not just because B can be written as the curl of A) but the phase of an electron is actually influenced by change in A is there an equivalent AB thought experiment for Dark energy Gamma?
A further thought on the existence of dark matter particles.
As I understand it, MOND (which I think everyone agrees is not a viable model but as an approximate, ad hoc model has had tremendous success–see the current post at Triton Station) postulates a cut-off point for Newtonian gravitational acceleration on the low end. This reminds me, in reverse, of the ultraviolet problem of classical black-body radiation, which led to quantum mechanics. That is, if we switch to discrete, quanticized math from GR’s continuous math, might that not somehow explain all the things we now explain with dark matter, just as quanticizing energy solved the UV problem?
Not that it is an easy thing to quanticize gravity, of course. Nobody has been able to do it satisfactorily so far. (Patience.)
I disagree somewhat with one point:
“We don’t know why the vacuum energy is much smaller than naive estimates would predict. That’s a real puzzle.”
Not all agree. It can’t be mentioned often enough: https://arxiv.org/abs/1002.3966
I would add another misconception: there is a flatness problem in classical cosmology. I would like to believe that the fact that you didn’t mention it as a misconception means that it is clear that it doesn’t exist, but I don’t think that we are there yet. Perhaps it means that you believe that it is true. For those interested, I recommend the excellent “review” by Marc Holman.
There seems to be some problem: comments not appearing, but no error message, and no mention of moderation. Sean, email me if you see this. (Other people: no need; if you see it, then I will too.)
Here’s a random thought. I was wondering why it is that dark matter does coalesce into “dark matter bodies,” analogous to stellar systems and planets.
Starting with John Baez’s discussion of the virial theorem, gravity, and entropy http://math.ucr.edu/home/baez/entropy.html
It seems that the difference between ordinary matter and dark matter is that ordinary matter can radiate, balancing the thermodynamic books, whereas it would be counter to the 2nd law for dark matter to coalesce beyond a certain low density. Just an odd thought.
I get quite confused about absolute dimensional datum when talking expanding universe. I still don’t get it, or perhaps it doesn’t make much sense.
One problem we have with physics is the inevitable conflating of concepts/variables implied by any physical law set out by mathematical relationship of parameters, i.e. any good physics. Mass and gravity, someone discussed. Which ’causes’ which? One may be the derived parameter in our particular equation or model, but that doesn’t necessarily say one is a product of the other.
‘What we see is not reality, but reality exposed to our mode of questioning’- Heisenberg.
Even a final reductionism solution would imply several (certainly 2) variables as far as I can see. What exactly are those variables envisioned to be, or to represent? Where did they come from? Science, if it gets that far, will posit these as ‘brute facts’.
Here, our mode of questioning is that of the scientific paradigm, more specifically the attempts to conform the material world to mathematical law. But we know this is falling to pieces and further progress is recalcitrant, at least for now, and with no final assurance it will get fully back on track. Feynman said we’d either get to the bottom of physics or make slow peelings back of the onion until it became too costly or impractical to proceed further. He assumed that it was inherently possible to make further progress indefinitely with the scientific reductionism paradigm. I don’t see that as necessarily true.
Another problem with over-emphasising the import of the physics paradigm on the total experience of life is highlighted by Anastasia Greenberg here. Grief can wrench people to the core. The feelings are intense and overpowering. The profundity of our emotional life as experienced points to a reality beyond our present physical one. For me the implications cannot be minimised with further clumsy ‘scientific’ explanations. Science is not that powerful or significant. It tells us a few useful things about how the world around us normally behaves. That’s often useful. That’s about as far as it goes. A lot of it (the views of the science community, not the method applied carefully) is speculative claptrap or orthodoxy dressed up as proven fact. Life is a whole lot more profound than all that. ‘Deep calls to deep’ as the Psalmist said. ‘Love is stronger than death’ as his son realised. Identity. Personhood. Conscious being.
Physics represents one way the world presents to us. Even on its own terms, it is manifestly incomplete. There are other ways reality presents. They are more subjective, certainly, and harder to reach consensus over. To very many of us, they shout louder about reality.
Dr. Carroll: First, thank you for your efforts to teach science on a broad basis. My question is, what will it take to get you physicists to stop using the word “pull” to describe gravity? It’s just so … Newtonian.
Dr. Carroll: First, thank you for your efforts to teach science on a broad basis. My question is, what will it take to get you physicists to stop using the word “pull” to describe gravity? It’s just so … Newtonian.
Perhaps because “push” is worse? Google “push gravity” to generate a list of crackpots.
As Mr. Larsson has already pointed out, “True Facts” is a tautology because there’s no such thing as a false fact: all facts are true by definition. This was pointed out to me about 55 yrs. ago by one of my math. profs. when I was an undergrad. Therefore, I recommend that you change your title from “True Facts about …” to “Facts about …”
Could dark matter actually be visible matter that gets cloaked as invisible, allowing light to pass around large swaths of a galaxy’s stars, by either unknown natural phenomenon or advanced alien civilizations? We have primitive invisibility cloaking tech for very specific frequencies. It’s not unfathomable that an advanced civilization could have perfected it.
the term ‘dark matter’ is a little misleading. it might very well be something mundane. its just not lit for some reason.