The previous 12 posts listed the main discoveries of modern cosmology in chronological order: putting all this information together leads to the Standard Model of cosmology (not to be confused with the Standard Model of particle physics). We conclude our short course with a simple overview of the standard model (also known as the Concordance Model). You will notice that it is also a brief history of time. Keep in mind that what follows is a model: the strength of the evidence for each phenomenon varies (see specific posts on each topic starting here).
1. The Big Bang
- The universe began approximately 13.7 billion years ago when it began expanding from an almost inconceivably hot, dense state. Ever since, the cosmos has been expanding and cooling, eventually reaching the cold, sparse state we see today.
- In the first 10-34 seconds, the universe experiences a brief period of extremely fast expansion known as inflation. This period smooths out initial inhomogeneities, leaving the universe with the homogeneity and isotropy we see today. Quantum mechanical fluctuations during this process are imprinted on the universe as density fluctuations that later seed the formation of structure.
- The infant universe is a soup of matter and energy in which particle/antiparticle pairs are constantly born and annihilated. As the universe cools, it becomes too cold to produce heavier particles, while the creation of lighter particles continues until temperatures cool to a few billion Kelvin. At this point, most of the remaining particle/antiparticle pairs are annihilated. A small amount of matter survives due to a slight asymmetry in the decay of between matter and antimatter.
- After a few minutes, nuclei of the light elements (hydrogen, helium and lithium) are formed by the combination of free protons and neutrons, a process known as nucleosynthesis.
- After about 100,000 years, the universe is cold enough for free nuclei and electrons to to combine into atoms (recombination). At this point, the universe becomes transparent due to reduced scattering by free electrons. Radiation now permeates the universe – seen today as the cosmic microwave background. By this time, dark matter (unaffected by the behavior of the baryonic matter) has already begun to collapse into halos.
- After a few hundred million years, galaxies and stars form, as baryonic gas and dust collapse to the center of the pre-existing dark matter halos.
A brief history of time
2. The Composition of the Universe
- Baryonic Matter: ~3% of the mass in the universe
This is ordinary matter composed of protons, neutrons, and electrons. It comprises gas, dust, stars, planets, people, etc.
- Cold Dark Matter: ~23%
This is the “missing mass” of the universe. It comprises the dark matter halos that surround galaxies and galaxy clusters, and aids in the formation of structure in the universe. Dark matter is believed to be composed of weakly interacting massive particles or WIMPs.
- Dark Energy: ~73%
- Observations of distant supernovae suggest that the expansion of the universe is currently accelerating. This observation is backed up by the flatness of the universe as measured from the cosmic microwave background. Cosmologists believe that the acceleration may be caused by some kind of energy of the vacuum, possibly left over from inflation.
Matter/energy composition of the universe
That concludes our short course in cosmology. You can find details on any of the topics above by scrolling through the last 12 posts of this blog. Alternately, you can find slides from a lecture I gave on the subject (The Big Bang – Theory or Established Fact?) by clicking here
There is a really nice one-page web summary of all of the above on the Talkorigins Archive here, and for readers requiring a slightly more advanced treatment, there is a good review of the current state of play in cosmology on the ARXIV here
25 responses to “The standard model of cosmology”
According to the BB theory, spacetime came to existence in the BB. As far as I know, it is accepted that it is only space that expands ever since. Are you familiar with a model (compatible with GR) in which the whole spacetime expands ever since?
No, Nirva, it is spacetime that is predicted to expand by relativity, and it is spacetime that we observe expanding (don’t forget the scale factor of the universe is plotted on the y-axis against time on the x-axis on Friedmann graphs).
Re origin of spacetime, that is a much more difficult question and probably won’t be solved until we have a decent theory of quantum gravity – see post on the singularity and the Hawking-Penrose theorems.
Thanks for your answer, but I am still a little bit confused… (I apologize, but my knowledge in cosmology is no more than that of an educated layman…)
As far as I know it is only the spatial slice of spacetime which is multiplied by a time-factor R(t) in the FRW metric. Hence the familiar interpretation of a 3-sphere, embedded in 4-d, and expanding as time goes by. As far as I understand (and I may certainly misunderstand) if the whole spacetime is expanding, the whole 4-d metric should be multiplied by some sort of ‘time’ rescaling function, but such metric might not solve the Einstein equations. I therefore concluded that it is just the spatial slice of spacetime which is expanding. Anyway, if the whole of spacetime is indeed expanding, how then do you observe (or measure) the expansion of time itself? what do you look for to see this?
Great Post Doc!
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I don’t think spacetime can expand – after all, what would it expand relative too? Some external time coordinate?
Spacetime just IS.
With the standard cosmological model we assume that:
1. spacetime is homogeneous (so a special time cordinate exists “cosmic time” where at each “instant” space “looks the same” (is isometric – i.e. equivalent metrics))
2. spaceime is spacially isotropic. (so “comoving” coordinate systems can be defined using cosmic time as time coordinate where rotating about the world lines of comoving observers is also an isometry.)
Do this and you (at least if you’re clever!) get the FRW metric and Hubble’s law. Hubble’s law says how the measurement scale, and hence proper distance, between comoving observers changes with comsic time.
If we use cosmic time as our time coordinate, and the worldlines of coomovers to define volumes in space then I suppose we could ask how these change with time. There are many subtleties here and counter-intuitive hazzards to be avoided.
Nirva – I always took it as time rolling out, forming a natural expansion in the 4th dimesion – must look this up.
James – excellent question. What does an expansion of spacetime really mean? Relative to what? To itself?This is a core question of GR, must do a post on it. But not right away, having got to the end of the course, I’m taking a break for a few days!
Well, somethings to mull over on your well-earned break…
Expansion implies change – change needs time for it to be defined. As a result, no self-contained universe can expand (or do anything else for that matter). Time is a part of our universe.
So, everyting is internal (I’m restricting myself here to classical GR – no fancy extra dimentions, gods, or theories of being in a computer simulation, for example…)
I am also no GR expert – wouldn’t even claim to be competent, so this is really an open question exercise. How can the universe expand? And who’s measuring?
By expansion we mean “space” in the crude sense, and the GR spacetime comes with a metric which we might use to compute the volume – but I can’t remember if this is possible since the metric is indeffinite and the manifold is very likey not compact. (Diff Geom long forgotten – must revise)
In more practical terms, I believe the notion of volume/distance comes down to red shifts. But even here we have trouble since I belive receding objects can be red-shifted and objects moving towards us can be blue-shifted (see arXiv:astro-ph/0104349v3)
So, everybody seems to know that universe expaneded from a tiny speck, but really knows how (or even what it means).
Well, could be just me not paying attention in the lectures…
Done some revision!
It seems it is standard to assume paracompactness for spacetime, which allows the integral to be defined, in particular it allows “partitions of unity” to be defined. (we also assume orientablity, and bunch of other things).
However, since the answer is likely to be infinite, this avenue is probably not very useful.
Observations of distant supernovae suggest that the expansion of the universe is currently accelerating. This observation is backed up by the flatness of the universe as measured from the cosmic microwave background.
Hi, I don’t understand this part, and I was hoping that someone might clarify it for me. How does the flatness of the universe “back-up” the supernovae evidence? A perfectly static universe is going to be flat, and a slightly off balance universe, like ours, will be flat, but an accelerating universe isn’t typically something that one would expect to be flat, so how does the statement make sense?
Island: the flatness of the universe backs up the supernovae evidence because a flat geometry indicates a perfect balance between the energy of expansion and the pull of gravity. Calculations show that there isn’t anywhere near enough density of matter (including dark matter) to balance the energy of expansion. One is forced to postulate the existence of energy that has nothing to do with matter – which concurs nicely with the postuate of Dark Energy from the supernova data.
Hi cormac, thank you for your reply, but now I’m even more confused than I was before you did… ;)
Island: the flatness of the universe backs up the supernovae evidence because a flat geometry indicates a perfect balance between the energy of expansion and the pull of gravity.
Right, flatness does not indicate that the universe is expanding, rather, flatness, in of itself, indicates that that the universe is not expanding, because it tells us only that the expansive tendency is perfectly balanced by the pull of gravity. “near”-perfect flatness tells us that the universe is expanding, ever so slightly, but again… nothing about acceleration, and certainly nothing to support it.
Calculations show that there isn’t anywhere near enough density of matter (including dark matter) to balance the energy of expansion.
Which indicates that the universe is not going to be flat for long, but again, flatness is not an indication of expansion.
One is forced to postulate the existence of energy that has nothing to do with matter – which concurs nicely with the postuate of Dark Energy from the supernova data.
Which is not supported by the observed flatness.
Island, I think you need to have a look at the 3 basic Friedmann models of the universe(flat open and closed). All of these are expanding universes at one stage – the only difference is in whether there is enough mass in the universe for matter to win out eventually.
In particular, a flat universe is also an expanding universe and it doesn’t close off. (An accelerating universe is simply a universe where the scale factor of expansion is increasing etc).
It comes down to the basic idea of dynamic spacetime – my favourite reference for this is Peter Cole’s book A Very Short Introduction to Cosmology (OUP)
Wow, no, we were talking about ***accelerating*** expansion.
Bottom line, an accelerating universe is not supported by flatness, as an accelerating universe is OPEN, because, as you said, “a flat geometry indicates a perfect balance between the energy of expansion and the pull of gravity”.
Without dark energy, a flat universe expands forever but at a steadily decelerating rate.
But with dark energy, the universe becomes less dense and the geometry of the universe becomes more OPEN as it accelerates.
Flatness does not support an accelerating universe.
FYI, I don’t mean to be disruptive and I am sorry if it seems like I am, but I commonly use the difference between a near-flat expanding universe and a wide-open expanding universe to make the point that one is geared more toward
entropic efficiency and “maximum energy”, since it wastes less energy to heat-death than an wide open universe will.
So I wanted to make sure that I have all my ducks in a row, and as of this moment… I do.
Apologies island, my answer was indeed confusing. I should have made it clear there are two separate issues here: flatness and acceleration.
While flatness is one possibility in Friedmann models, (and a flat universe expands), an accelerated expansion is not predicted by any of the Friedmann models. This is an important point, and not emphasised enough.
Now we discover experimentally that our universe seems to be both flat and accelerating! What does this mean?
No-one knows yet, as far as I can make out. I gather that flatness and acceleration are in fact compatible in the context of some models of inflation – but there is much work to be done on this.
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Ah – of course, I should have correctly said that receding galaxies can be BLUE shifted and approaching ones RED shifted (otherwise, that would not be a very remarkable fact…)
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What is considered, in the current standard model, the physical cause for the expansion of the universe? What should it explode? Explode against what counterpressure?
How come gravity didn’t prevent it while density was almost infinite?
Are this questions addressed by the standard model?
A very good question indeed Enrique.
The basic cause of spacetime expansion is quite difficult to understand at the phenomenological level – the mathematics of general relativity simply predicts that spacetime is dynamic, not static.
For example, even in a universe with no matter at all (deSitter universe), spacetime is dynamic – in fact it would expand without limit because there is no gravitatinal attraction of matter to counteract the expansion!
Related questions are what is the physical cause of the exponential spacetime expansion of inflation? Or today’s slightly accelerated expansion? Anyone’s guess as far as I can see…
I am a laymen with an interest in the subject. I was surprised to see that there seems to be a concensus that the expansion of the universe is accelerating, I thought the observational evidence led one to believe the expansion is decelerating. I am certain my view is simplistic, however, most that I have read seems to indicate that the most distant objects are receding slower than objects that are closer based on red shift, considering gravitational lensing. My impression is that these observations are based on standard candles (cephied variables), measured relative to their apparent brightness, to gauge relative distance, and then looking at the amount of shift vs the apparent distances involved. An ever expanding universe is probably more interesting in as much as it rules out a Big Crunch and gives life a clear direction in terms of crafting survival strategy in the distant future. Please help me to understand the observations and theory that conjectures accelerating expansion.
Hi Micheal, no, the evidence from supernove measurements and from the cosmic microwave background suggest that the expansion is currently accelerating (due to something we now call dark energy). That said, it has also been discovered that the expansion was deaccelerating just a few million years ago – and no-one is quite sure why dark energy is suddenly dominating!