I was surfing the web the other day, and I came across Sean Carroll’s Preposterous Universe blog. There’s a post on there called True Facts About Cosmology (or, Misconceptions Skewered). I thought it looked interesting, because that’s my kind of thing. I do so dislike all the popscience and the lies to children, and it pleases me to see somebody else trying to put the record straight. But has he? I thought I’d take a look and give my opinion. Carroll leads in with something about entropy. He’s blue, I’m black:
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.
Thermodynamics leaves me cold. So I like to think of entropy in terms of energy dispersal. So I think of entropy as “sameness”. On top of that, I suspect the early universe was something like a frozen-star black hole. That would mean the entropy was high rather than low. But heck, I’m guessing about the early universe. There isn’t a lot to go on, and I for one don’t know why the universe started expanding. Anyway, here’s Carroll’s list:
1. 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.
We have good evidence that the universe is expanding. Even if we didn’t, I’d be saying it just has to expand. That’s because I think of space as being like some kind of compressed gin-clear ghostly elastic. But I note that when the CMB was discovered in 1965, it was described as evidence for the Big Bounce. The two famous back-to-back papers were A Measurement of Excess Antenna Temperature at 4080 Mc/s by Penzias and Wilson, and Cosmic Black-Body Radiation by Dicke, Peebles, Roll, and Wilkinson. Whilst both the Big Bang and the Big Bounce feature a hot dense state, they’re so very different that I wonder if the overwhelming evidence could also be overwhelming evidence for something else. Especially since time dilation doesn’t feature in either model. Especially since the standard Big Bang story features a rather unsatisfactory creation ex-nihilo. So I don’t quite agree with this first item. But maybe I’m splitting hairs about the word “overwhelming” here. So moving swiftly on:
2. 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).
I don’t think there are any point-singularities inside black holes, and I don’t think of the Big Bang in terms of a point singularity either. On top of that I definitely don’t think it’s a singularity of infinite curvature. A gravitational field isn’t curved space, it’s a place where space is neither homogeneous nor isotropic. Spacetime curvature is where the inhomogeneity changes in a non-linear fashion. So a homogeneous universe is a flat universe. So space is flat now, it was flat a billion years ago, and it was flat a billion years before that. It’s always been flat. As for the Big Bang being a moment in time, what about my time dilation? Surely it would have been infinite, which causes an issue for that “moment in time”. But no matter, Carroll says it’s hypothetical and probably not even true, so we’re on the same page here.
3. 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.
I suppose they do. They say things like “at the time of the big bang” and they mean the early universe. I don’t think there’s any problem talking about the hot dense state circa 14 billion years ago. However was it hot before that? A hot electron is a fast-moving electron, which was probably created along with a positron out of photons in gamma-gamma pair production. But where did the photons come from? What banged? Was it hot before it banged? Or did space ring like a bell and was suddenly full of waves and pair production and annihilation and fusion and everything else? I don’t know, and I’m not sure I’ll ever know. Next:
4. 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.
Hey, that’s what I said! It’s refreshing to hear somebody saying this sort of thing. We don’t really know. When I was a kid I didn’t accept God did it, and now I don’t accept a quantum fluctuation did it. What did it? Pass.
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).
This is good. I just hate those fairy tales about virtual particles popping into existence. How can there be any place for that sort of thing in science? So I hate the upscaled version that says the whole universe popped into existence. So I like what Carroll is saying here.
6. 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.
That’s good too. Like I said, I don’t like point singularities. I’m happy with the Schwarzschild singularity at the black hole event horizon, and I’m happy with the torsion string singularity in the Falaco soliton. But a gravitational field is a place where the speed of light is spatially variable. At the black hole event horizon the “coordinate” speed of light is zero, and it can’t go lower than that. So there’s no more gravity. So I don’t think there’s a point singularity at the centre of the black hole. So I don’t think there was a point singularity associated with the Big Bang.
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.
Another point that gets the thumbs up from me. I remember Stephen Hawking saying the universe came into being because of a quantum fluctuation. The obvious question is what fluctuated? Hawking didn’t get much criticism on account of his medical condition, so I was a little surprised to read M-theory, religion and science funding on the BBC in physicsworld in 2010. It talked about scientists cringing because Hawking was making a sweeping statement based on his faith in an unsubstantiated theory. I took careful note of that. What’s next?
8. 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.
Ah, sorry Sean, but I’m sure this is wrong. Gravitational field energy is positive, not negative. That’s why Einstein said “the energy of the gravitational field shall act gravitationally in the same way as any other kind of energy”. When two planetoids fall together and coalesce, gravity converts potential energy, which is mass-energy, which is internal kinetic energy, into external kinetic energy. This tends to get radiated away, and you’re left with a mass deficit. But there is no energy deficit. Conservation of energy applies. Ditto if you pull your planetoids apart and separate them. You do work on them. You add energy to them. You increase their mass. Wikipedia tells us that the zero-energy universe was proposed in 1973 by Edward Tryon, who also proposed that the universe was born of a quantum fluctuation. His paper appeared in Nature, and was called Is the universe quantum fluctuation? Interestingly it says “the prevailing attitude towards creation is that our Universe is a probably undergoing merely one of infinite series of expansions (with intervening contractions)”. That’s the Big Bounce. The paper also employs a non-sequitur argument about a closed universe having no external gravitational flux when viewed by an outside observer, and therefore it must have zero energy. Both Alan Guth and Stephen Hawking repeated the myth of the zero-energy universe. See page 13 of Guth’s 1998 book The Inflationary Universe, and see page 82 of Hawking’s 2002 book The Theory of Everything.
9. 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.
I think this is wrong too. Electrons are not losing energy due to the expansion of the universe. Nor are protons or neutrons. Nor are photons. Yes, I know people talk about CMBR being redshifted by a factor of a thousand, but IMHO it’s a myth. It’s related to the myth of gravitational redshift. The ascending photon doesn’t lose any energy. Instead, like Einstein said, “an atom absorbs or emits light at a frequency which is dependent on the potential of the gravitational field in which it is situated“. Check out the Scientific American article Is the Universe leaking energy? It features Tamara Davis who talks about galactic redshift as a Doppler shift. I think she’s right, and that the same applies to the CMBR photons. They haven’t really lost any energy.
10. 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.
Call me a nit-picking pedant, but spacetime models space at all times, so spacetime isn’t getting bigger. Space is getting bigger. But OK, the universe isn’t expanding into anything. There is no space outside the universe. Of course, I think the universe has an edge, but don’t mind me.
11. 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.
Good one Sean. I’ve had a good look at inflation, and I’d give it a 0% chance of being correct. The monopole problem misses the point that the electron has an electromagnetic field, the flatness problem misses the point that homogeneous space is space without a gravitational field, and the horizon problem misses the point that the universe may have started with no temperature at all. It means inflation is a solution to problems that do not exist.
12. 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.
I’m not sure what Carroll’s saying here. People like Hawking have said the universe can be likened to a black hole in reverse, which I think is fair enough. But if that’s a frozen-star black hole, everything’s the same, so entropy is high. Like it’s said to be for a macroscopic black hole. And since everything’s the same, the early universe is smooth, not lumpy. OK it’s lumpy once the Big Bang bangs. Then I suppose it looks like a thermal gas. But what’s that about gravity? A gravitational field is inhomogeneous space, typically made more inhomogeneous when a non-uniform distribution of matter coalesces from dust and hydrogen to form a star. So the universe is getting lumpier as it gets bigger. Gravity is reducing entropy, but it results in stars which are increasing entropy. Which leads to the high-entropy heat-death universe where the sameness is back but now the space is much bigger. Nevermind. What’s next?
13. 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.
Yep, dark matter exists. I can’t argue with that. What I do argue about is the presumption that dark matter consists of particles. Sometimes I feel like I’m the only one who’s read the Einstein digital papers. I feel like I’m the only one who’s taken note of “the energy of the gravitational field shall act gravitatively in the same way as any other kind of energy”. That’s inhomogeneous spatial energy. If you’ve got some region of space where the spatial energy density is higher than average, the extra energy must surely have a mass-equivalence and a gravitational effect.
14. 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.
Patience? When WIMPs were first proposed in 1977? See David Spergel’s 1998 article on particle dark matter which refers to Piet Hut’s limits on masses and number of neutral weakly interacting particles. WIMPs have been around for forty two years now, getting in the way of inhomogeneous spatial energy that’s bound to occur in a raisin-cake universe. Space expands between the galaxies but not within, so every galaxy is a place where the cake is denser than average. Meh, I think WIMPs are a perfect example of supersymmetric pseudoscience getting in the way of real physics.
15, 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.
I suppose it’s fair enough to say dark energy may not exist. The Wikipedia dark energy article gives what I think is a reasonable definition: “dark energy is an unknown form of energy which is hypothesized to permeate all of space, tending to accelerate the expansion of the universe”. Something is causing the universe to expand faster and faster, but it might not actually be energy doing this. I say that as somebody who likes to think he knows what energy is. However I don’t think it’s fair enough to say the expansion of the universe is caused by gravity, modified or not. Ditto for the accelerating expansion of the universe. That’s because a gravitational field is akin to a pressure gradient in space, whilst expansion is to do with pressure and tension. The pressure of the air inside a balloon in vacuo is counterbalanced by the tension in the skin, and you can make the balloon expand either by increasing the pressure or reducing the tension.
16, Dark energy is not a new force; it’s a new substance. The force causing the universe to accelerate is gravity.
Like I said, the force causing the universe to accelerate is not gravity. Like I said, a gravitational field is akin to a pressure gradient in space, whilst expansion is to do with pressure and tension. I like the analogy of a bubble-gum balloon in vacuo when thinking about accelerating expansion. There’s pressure in the balloon, so the balloon expands, so the skin gets thinner, so the tensile strength reduces, so the balloon expands further, and so on. I should make it clear that dark energy is said to be responsible for the accelerating expansion of space. The phrase was coined by Michael Turner in 1998, in a paper about the supernovae observations that provided evidence of the accelerating expansion of the universe. The trouble with that, is what’s responsible for the expansion of space? Hence I side with what Ramesh Gupta and Anirudh Pradhan said in their 2010 paper Is Hubble’s Expansion due to Dark Energy? It’s responsible for both for the expansion of space, and the accelerating expansion of space. Is it a substance? I’d say dark energy is both the innate cosmic pressure of space, and the reducing tensile strength of space, and that space is the substance. But I think of energy as a volume of stressed space, and space is dark, so I’m fine with dark energy being a substance.
17. 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.
Whilst I don’t think the cosmological constant can actually be constant, I’m happy with an energy inherent in space itself. Like I said, I think of energy as a volume of stressed space. Take all the energy out of a wave, and it’s not there any more. Take all the energy out of space, and its not there any more either.
18, We don’t know why the vacuum energy is much smaller than naive estimates would predict. That’s a real puzzle.
I would venture to say vacuum energy is smaller than naïve estimates would predict because those naïve estimates are wrong. See Svend Rugh and Henrik Zinkernagel’s 2002 paper on the quantum vacuum and the cosmological constant problem. In the 1920s Wolfgang Pauli’s did a “café calculation” and said zero-point energy would result in a universe that “would not even reach to the moon”. In 1948 Niels Bohr said zero-point energy “would be far too great to conform to the basis of general relativity”. These were naïve estimates from anti-realists who didn’t understand the photon, the electron, electromagnetism, gravity, or anything else. They didn’t understand that vacuum fluctuations aren’t the same thing as spatial energy. The energy of the gravitational field is nothing to do vacuum fluctuations.
19. Neither dark matter nor dark energy are anything like the nineteenth-century idea of the aether.
I beg to differ on this one. In his 1920 Leyden Address Einstein referred to space as the aether of general relativity. He thought of a field as a state of space. Since dark energy is, according to Carroll, a substance, I think dark energy is like the nineteenth-century idea of the aether. Ditto for dark matter. If you’ve got some region of space where the energy density is higher than average, the extra energy must surely have a mass-equivalence and a gravitational effect. So I’d dark say energy isn’t a new substance, it’s an old substance. The fifth element. The quintessence. But in truth it’s the first essence, and the only essence. Because it’s the only thing. The one thing we can neither create nor destroy. The thing from which all other things are made.
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.
That’s it. Overall I think it was a pretty good list from Sean Carroll. There’s only three out of nineteen items that I didn’t like, and plenty of other that I did like. Maybe I should do my own list of misconceptions? I fear it might take a while. But it would be fun if somebody gave some feedback to that.