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.
Dear Dr. Sean Carroll,
WoW! I am glad we are all thinking about the origins of this, our Universe in physical terms. It’s when we do not, or do not specify what is our scientific knowledge and our conjectures based on that scientific knowledge we all can be led astray.
We do not know what the initial laws of our Universe were. It is known from the experiments at CERN, it must have had very different laws before electrons and atoms; even before matter itself was created. Did it start out as 2D, or 5D ? Were there two dimensions of time or always one. Or was there a Universe with no dimension in time. We do not know, or even if such competition and physical “excitement” exists today. We must all agree we are in a mundane 4D Universe until proven wrong. It is stable where certain things exists (energy, spacetime, etc.) and that it is evolving and evolved from something else, and my be re-born again or dissolve completely. It certainly does not look like that Green/Purple/ trampoline you have pictured from NASA where galaxies look the same, but gravity and warping exist. How absurd is that? All the best colleague, Dr. Kenneth M. Beck
The only thing I didn’t understand in this excellent post is in #16 when you say “the force causing the universe to accelerate is gravity”. I don’t understand that at all. I think the reason it’s baffling is obvious, but in case it’s not: I’m thinking gravity should oppose the expansion of the universe, since it is attractive. What am I missing?
Dear Dr. Carroll,
Thank you for this blog. I have also recently purchased your book on the nature of time which I am reading now.
I have a perhaps unusual and deep question for you, if you would be kind enough to respond. I am a scientist too (just not as smart as you) and I hold a PhD in Neuroscience. Given this background in biology, I believe firmly, like you, that death is the end of consciousness.
I recently lost my grandmother and have entered an existential crisis which has led me to search for answers in physics to deep questions. I want to understand how we can best think about the nature of time (which is why I bought your book) in order to make sense of life and death as events that occur with time. After my grandmother died, it hit me that now that she is no longer alive, she no longer knows she had a life, a career, and a grand daughter. What is the point of life if we all cease to exist one day? This question made me think deeply about time. Is the past real? Is it “out there” somewhere in space-time? If one moment turns to the next and eventually we all die and our species becomes extinct, then why bother with anything?
I understand if these questions are a bit too philosophical for an answer, but in my desperation I decided to leave these questions here. I do not have religion to turn to to create meaning out of life and so I am turning to other scientists. How do you make sense of your existence and purpose in life and how do you deal with the inevitable death of everyone that you love (including yourself)?
Thank you for reading this 🙂
Thanks Sean, very interesting.
Just one small misconception to add. ‘Space’ references STATIC existence/matter – static because, by definition (i.e. ‘Space’ has only 3 dimensions) Space excludes Time and hence excludes change.
Time references change (we know this because change is the only empirical evidence we have that ‘Time passes’). It does this in two distinct contexts:
1. The ‘dimension’ i.e the abstract framework (mental model??) we use to calibrate and index change and change events (dates, ages etc etc based on our standard units);
2. The’ flow’ – i.e. the mass noun – the non-specific set of change/change-events. For example, the word Traffic is a mass noun too. Traffic is to Vehicles what Time is to Change – a non-specific collection. Does Traffic exist, fundamentally?? Or just vehicles? Traffic is just a word – vehicles are the fundamental reality. Time is just a word, Change is the fundamental reality.
And all change is caused by energy differential, as you know.
So, merge the two and their offspring Space-Time references DYNAMIC everything. But it too must be abstract (per parents). The underlying fundamental is Existence/Change – in other words Matter and Energy (differential) are the universal fundamentals; Space and Time are merely abstract frameworks (i.e. that only exists in our collective minds) to reference the underlying reality of Matter and Energy [energy differential actually 🙂 ]
[e.g. try thinking of The Arrow of Change rather than the Arrow of Time…it helps. Change is specific, Time is general].
So Time doesn’t begin or end – Change does. And Space doesn’t begin or end – Existence does.
Just a little bit of lateral ‘out of the box’ for you…
If the universe is infinite now, it must have been infinite at the time of the Big Bang. If so what implications does this have for a Big Bang that emerges out of a singularity?
On #14 concerning the nature of dark matter, I would like to take issue with the potential implication that we “don’t yet have the technology to look for” dark matter other than WIMPs. Black hole dark matter theorists (who this past Tuesday took first place in the 2018 Buchalter Cosmology Prize) have made a number of predictions, such as that “between 0.1% and 1% of the events detected by LIGO will involve a PBH with a mass below the Chandrasekhar mass, which would unambiguously prove the existence of PBH” as per Clesse and García-Bellido, in arXiv:1711.10458, “Seven Hints for Primordial Black Hole Dark Matter.”
I really do like Clesse and García-Bellido. Their arXiv:1812.11011 from last month discusses PBH clustering as an answer to the only micro-lensing constraint agains black hole dark matter. It is so satisfying how Espinosa, Racco, and Riotto in their Buchalter Prize winning arXiv:1710.11196 show that if PBHs arose because of Higgs vacuum instability, then they would probably be clustered. That in turn provides an easy path to SMBH assembly without accretion of impossible amounts of baryonic matter. Clesse and García-Bellido have also been completely unafraid to correspond over email, while WIMP proponents are much less so in my experience.
Great post. But I just want to second Andrew’s question above about 16. How does gravity cause the universe to accelerate?
The ongoing null results at the LHC ( except for the Higgs of course) has been an inspiration for Neil Turok, Kieran Finn and Lathan Boyle to write two interesting papers, ” “CPT -Symmetric Universe” and “The Big Bang , CPT , and neutrino dark Matter.” I mention this because this set of ideas includes an attempt to get rid of the multiverse prediction by not invoking cosmic inflation to explain the Isotropic, homogeneous and spacial flatness of the universe. It does not appear they actually present a complete argument in these two papers of how this can be done but there is a promise of a more complete explanation in a future paper. The motivation seems to be the notion that the universe is much simpler than suggested by all the models beyond the standard model, like SUSY, GUTs and of course Super String -M theory. Dr Vic Stenger , long before the null results at CERN, had made a remarkably similar argument for simplicity. ( Vic passed away in 2014) Of course the door is certainly not closed at the LHC for some surprises , so one shouldn’t be so confident in thinking this way.
Gravity causes the universe to accelerate because gravity is not always attractive. Roughly speaking, the “source of gravity” is the energy density of a fluid plus three times the pressure of that fluid. Ordinary substances have positive energy and pressure, so gravity attracts. But vacuum energy has negative pressure, equal in size but opposite in sign to its energy. So the net effect is to push things apart.
Thank you Sean for these reminders.
In your Point 2, “The Big Bang … [is] a classical prediction, ignoring quantum mechanics”, actually reinforces the preceding “… not a point in space, but a moment in time …”. Indeed, when quantum mechanics is not ignored, neither space nor time need to exist, as anything unmeasured need not exist.
One way to illustrate that is to think of the Big Bang as a possible “white hole”. The latter being some reversed black hole. The black hole and the white hole could be linked, at least in one calculation per loop quantum gravity, by quantum tunneling (A. Ashtekar, et al. “Quantum extension of the Kruskal spacetime”, Phys. Rev. D 98, 126003, 2018).
If true, this quantum tunneling is equivalent to a classical wormhole. Both are linking up spacetime regions, albeit with different designations. A wormhole straddles two black holes whereas quantum tunneling connects a black hole with a white hole.
To the extent a wormhole involves quantum entanglement, and the latter being an integral part of any quantum measurement, we are actually back to the beginning of this comment. The Big Bang is the moment in time when classical spacetime comes into being, via a quantum measurement.
Just as one cannot predict where on the detection screen a photon will land in a typical double-slit experiment, one caveat of the above scheme is that beforehand, no two spacetime regions have prior knowledge of each other (https://quantumfrontiers.com/2018/05/20/a-quantum-podcast/#comment-98046). As such, a black hole has no control over which spacetime region will host the Big Bang white hole.
Thanks Sean! Sounds like I need to do some reading on vacuum energy and the effects of pressure on gravity.
Nice, though “true facts” sounds tautological.
In these types of lists where will be unclear or arguable points, while satisfying everyone will be impossible anyway. But for what it is worth for readability for or dialog with non-cosmologists, here are my problems:
#4: The Planck LCDM model is the current best (integrates most data), and it has an inflation period before the Hot Big Bang state of #1/older cosmologies. And since it subjected the universe to an adiabatic process, it was cold and quite empty as far as I understand [ https://profmattstrassler.com/articles-and-posts/relativity-space-astronomy-and-cosmology/history-of-the-universe/inflation/ ].
#8: The recent 3d data release of Planck LCDM data has the universe as extremely flat, not closed. (But it would be interesting to know why zero energy is also believed to apply to closed universes.)
#11: Again, if Planck LCDM is the reigning cosmology, inflation has insufficient contenders. I guess that explains the “many” cosmologists that accept it as a fact. But as fact it would be more robust than simply specialist consensus, since DES later confirmed Planck’s 3d release, and before that many other tests has agreed.
#15: You know the drill, Planck et al says dark energy exist, and is most likely the vacuum energy.
#18: And again, Planck 3d data release rejected eternal chaotic inflation but passed eternal slow roll inflation. In its unconstrained, most likely form that process results in that we live in a universe selected by survivor bias. So it is not a puzzler as such, as much as arguable and hard to test.
Hi Sean,
Based on the comments of the readers, I guess an obvious candidate for your list is:
20. Gravity is attractive only in the nonrelativistic low-energy (Newtonian) limit, while outside of that limit it can be either attractive or repulsive, depending on details.
Apparently most laymen still understand gravity only in the Newtonian sense, and are not aware how large a modification GR introduces to it.
Best, 🙂
Marko
@ Anastasia Greenberg:
“Given this background in biology, I believe firmly, like you, that death is the end of consciousness.”
Speaking of facts, I believe this has been a (possibly arguable) fact since 2017. There were two or three earlier areas that implied it by consistence – medicine (anesthetics: why chemical effect), biology (evolution: when did it evolve), possibly thermodynamics (information: so much synaptic added heat in a human brain) – but when LHC theory and cross checks were done several physicists (Sean one of them) went public on this (too little remaining interaction for ‘ghosts’ et cetera).
“What is the point of life if we all cease to exist one day?”
The point of life – and death – is obviously to me, as a new bioinformatician, to propagate the evolutionary process. But the point or “purpose” of being a human individual is arguably what we decide for ourselves.
@Hanko van Beinum:
So I’m no cosmologist, but let me try based on earlier physics studies and geeking out on cosmology: we don’t know that the universe is infinite. (But observing an “eternal” slow roll inflation and flat space makes it most likely – the less constrained alternative – I think.)
But there is one thing I have read many times, from Sean among others, that they want to observe the comoving volume, I guess to extract as much information as possible though they express it as something they should (or have to?) do. So we can see cosmologists state that the universe appears closed and its topology as mostly approximate to deSitter conical section with a Planck size bottleneck. I don’t get that since I am not a technical expert: if volumes all the time transition from “smaller than Planck minimal volume” to “larger than Planck minimal volume” as the universe expands signifying a rescaling based on the smallest distance that makes sense, there should be no problem of principle. An infinite universe today was always infinite as you say, even under the Hot Big Bang moment in time, you have “just” (sounds simple, does it not) to let go of the comoving volume of the observable universe and look at ever larger volumes.
Anastasia– I’m not sure which book you bought, but The Big Picture talks about this a bit. I think the question “What is the point of life if we all cease to exist one day?” is not the right one to ask. It’s analogous to “Why should I eat this meal if I’ll just be hungry again later?” or “Why should we play this game if we know it’s just going to end?”
There is no overarching, objective “point” to life. There is only the point, or whatever set of many points, we personally decide to attribute to our lives. Yes, someday I will be dead and there will be no “me” to remember what I did while alive. But I’m alive now, and what I do matters a lot, at least to me. That’s the starting point, and individual people will go in different directions from there.
Thank you for your response, Sean.
Maybe my current state is just grief and I am trying to find my way out with science and logic. I find grief to be very challenging to navigate when seemingly everyone I talk to has spiritual or religious views about an afterlife. Of course, this also goes back to the nature of time because I would ask them: “How long does the “afterlife” last? What happens after the “afterlife”? Does time go on forever?” Taking the position that this is the one and only life that we have, I am on a quest to better understand time itself since my life exists “now” and for people who have passed, their lives existed in the “past”. This is the first time in my life that I have really begun to appreciate how deep the concept of time really is and how non-obvious it actually is.
I bought several of your books so I hope to discover something there. Thanks for taking the time to respond to me.
Sean,
There are a couple of issues I have with the psychology and logic on which cosmology is based.
First, that as mobile organisms, we evolved a sequential thought process, in order to navigate, so the narrative of time is foundational to our sense of self, culture and civilization, but the logical basis of this past to future effect is change turning future to past. Potential>actual>residual.
Duration is not evidence of some underlaying dimension, but the present, as events form and dissolve.
There is no physical past, because the present consumes it, in order to be informed by it, ie. cause and effect.
Different clocks can run at different rates because they are separate actions. The turtle is still plodding along, long after the hare has died, because it has a slower metabolic clock rate and no space travel is necessary.
Time is asymmetric because action is inertial. The earth turns one direction not both.
That different events appear in different order, from different locations, is no more consequential than seeing the moon as it was a moment ago, simultaneous with seeing stars as they were years ago. It’s the energy that’s conserved, not the information. It’s the change of the information being manifest by the energy that creates the effect of time. The energy is always and only present, which is why it’s “conserved.”
Another problem I have is that when it was first discovered that redshift is the same increasing rate in all directions, the theory was changed from an expansion in space, to an expansion of space, based on the premise of spacetime. Yet that totally overlooks the fact that if the light is being redshifted, thus taking longer to cross, it’s obviously not Constant to intergalactic space. Two metrics are being derived from the same intergalactic light. The rate the inchworm crawls and the balloon expands are based on the speed and spectrum of the same light. Which is the denominator and which is the numerator? If light speed is the ruler, the expansion is still just distance.
We are at the center of our point of view, so possibly an optical effect might be worth considering? While single spectrum light is only redshifted by recession, multi spectrum light is also redshifted by distance, so possibly those photons hitting telescopes haven’t traveled individually for billions of lightyears, but are samplings of a wave front. It might better explain how we can get so much information out of them.
One last point, a basic premise of science is falsification, but 30 years ago, when Perlmutter et al, found a gap between prediction and observation, there was no questioning the theory, it was simply assumed that 70% of the content of the universe is invisible to everything, but this gap. Why is that?
If redshift is optical, the curve in the rate would be explained by this effect compounding on itself and going parabolic, until galaxies seem to be receding at the speed of light, with everything beyond that shifted off the visible spectrum. So the cosmic background radiation would be the solution to Olber’s paradox. The light of infinite sources.
Anastasia,
Our spiritual paradigm has been dominated by monotheism for some millennia, but the logical fallacy of that is a spiritual absolute would be the essence of sentience, from which we rise, not an ideal of wisdom and knowledge, from which we fell. More the emotion and awareness of the new born, than the knowledge and judgement of age. The Ancients tended to build their religions around nature and emotion, but we have this top down, father figure lawgiver and his Ten Commandments. Which, not coincidently, validates authority and order over everything else. While god might be dead, we still live in its shadow.
Which is not totally to answer your situation but to try removing some of the cultural baggage. We are all of this essence bubbling up, but it crumbles back down. The price we pay to feel, is that much of it is pain.
Dr Carroll,
i would like support Andrew’s suggestion that we need from you a nerdy layman’s exposition on gravity — perhaps a stand alone blog expanding items 8,9, 14 and 16. It sounds like many of us need a better mental model of gravity in the form GR would have us see it… and some discussion of related tho yet undecided phenomena: I mean for instance, what happens to the notion of attractive-then-repulsive gravity if it is decided there are gravitons ( particles responsible for the force of gravity)? And perhaps some math would help us get the notion of zero total energy in an ever more quickly expanding universe, straight. Repulsive gravity? Damn. (Maybe you would recommend a textbook at least.)
Dr Carroll I have just purchased your book ” Spacetime and Geometry” GR is never easy going, especially for those of us with limited Math skills but your book is one of the better ones. I already have notes and underlines all through it. It will be useful for me.
Very helpful. Thanks Sean.
I’m seconding @vmarko’s suggestion:
20. GR is a REALLY large change from Newtonian gravity.
Hell, even I didn’t realize this until this Twitter thread.
-Ryan
Sean Carroll says:
January 12, 2019 at 2:49 pm /
Gravity causes the universe to accelerate because gravity
is not always attractive. Roughly speaking, the “source of gravity”
is the energy density of a fluid plus three times the pressure of that fluid.
Ordinary substances have positive energy and pressure,
so gravity attracts. But vacuum energy has negative pressure,
equal in size but opposite in sign to its energy.
So the net effect is to push things apart.
#
SelfAwarePatterns says:
January 12, 2019 at 2:57 pm
Thanks Sean! Sounds like I need to do some reading on vacuum
energy and the effects of pressure on gravity.
Doesn’t what mean –
that Sean Carroll’s negative vacuum energy (as whole) is stronger
than positive gravity (as whole)
===
I’ve been led to believe that the Universe began with minimum entropy – a near absence of heat, a supersaturation of primal ingredients as orderly as a multidimensional house-of-cards (energy fields), and as precipitously balanced as well. Space-time emerged as this supersaturated house-of-cards began its cataclysmic precipiation – allowing “heat” to arise – chaos! Supersaturated states need seeds to initiate precipitation … what would be the seed for this cascade? Resonance and standing waves are known to take structures down. Hence, the Big Bang denotes the beginning of Space-Time
Dear Prof Carroll,
Regarding the perceived size of the universe, frequently I read science writers refer to “the universe” as a finite space with finite galaxies and matter. Unless I am mistaken, in this context they should be discussing the “observable universe” as finite. What’s more I often read descriptions of early galaxies at the edge of the observable universe that are approaching the max distance of 13.7G l-yr. While light did travel that distance of space, many people naturally assume that that would imply that these galaxies are now nearly 13 G l-y away. What’s more, people assume that these galaxies are 13 G l-y away today rather than at nearly 47 G l-y due to expansion since that light was emitted. Should it not be made clear that the estimated diameter of our observable universe is now around 94 G l-y and those early galaxies were a lot closer to where the Milky Way is now when they emitted their light long ago?
-Rob