So scientists are finding that the Universe is expanding at an accelerating rate. It is said 'Dark Energy' is the cause. Ok. But the Universe expanded at a reaally accelerated rate just after the Big Bang too! Presumably at a faster rate then 'now' (whatever time the observations stem from)

So, in-between 'now' and the Big Bang it must have slowed down no? Or else the universe would currently be expanding at an even faster rate than at the Big Bang.

So what happened in-between? If 'current' expansion is really caused by this Dark Energy, why wasn't that in effect in the in-between time? What was going on then?

Shouldn't we be able to measure the expansion at different eras/ages?



4 Answers 4


The expansion of the Universe is driven by the dominant forms of energy and matter density. This is essentially an expression of a famous statement by John Wheeler (from memory, apologies if I get the exact wording wrong): "Spacetime tells matter how to move, matter tells spacetime how to curve". In this case, the matter is telling the Universe how to expand. Mathematically, Einstein's equations describing gravity, when restricted to homogeneous and isotropic Universes (relevant for cosmology), reduce to the Friedmann equations which relate the expansion rate to the matter content.

There are lots of different kinds of energy density, but for cosmology there (to zeroth order) three that really matter:

  • When the Universe is dominated by non-relativistic matter (atoms and molecules but also dark matter), the size of the Universe grows with time as approximately $\sim t^{2/3}$.
  • When the Universe is dominated by relativistic matter (such as photons or a very hot gas of particles moving relativistically), the Universe grows as $\sim t^{1/2}$.
  • When the Universe is dominated by an approximately constant energy density (more on this in a minute), the Universe grows exponentially as $\sim e^{Ht}$, where $H$ is the expansion rate.

Note that ordinary matter (relativistic and non-relativistic) do not have a constant energy density. As the Universe expands, the energy density (energy per unit volume) goes down, because there are the same number of particles but a larger volume. (Well, it can be that the number of particles changes, but at the level of this discussion that's a detail that is not so important, the key thing is that the energy per unit volume decreases for ordinary matter).

So something weird is needed in order to provide constant energy density as the Universe expands. Physically we can imagine that there is a certain amount of energy associated with every volume of space -- then as space expands, in a sense creating more space, there is also more energy associated with that space. This is essentially the cosmological constant. Another possibility is that there is a dynamical field (a "scalar field") which permeates space, and an associated potential energy that is approximately the same anywhere.

With all that background, the current generally accepted model of cosmology is that:

  1. The energy density is initially dominated in some scalar field, whose large potential energy permeates space and causes exponential expansion.
  2. The scalar field rolls down its potential, and eventually rolls to an area of its potential where its potential energy is very small. During this process, the scalar field transfers its energy to ordinary matter and radiation fields.
  3. The Universe enters a phase where it is dominated by relativistic, hot matter.
  4. Eventually the Universe cools to a point where the matter that dominates the energy budget is non-relativistic, so the growth rate changes.
  5. As the Universe cools even further, and its energy density lowers, eventually the energy density of the Universe becomes dominated by a small constant value that was likely present all along, but (up until recently) was only a negligible contribution to the overall budget. This last component is what was discovered in the late 90s as the accelerated expansion of the Universe.

While this is sort of a standard picture, not all points are on equally solid footing. The first two bullet points are not universally accepted. The first bullet point is called "inflation", and (if true) explains why the Universe we see is spatially flat, and why we observe the CMB to be at the same temperature, even though the different patches that make up the CMB were not in causal contact with each other before the Big Bang in models without inflation. The inflationary period also explains the origin of the spectrum of the CMB. The second bullet point is a mysterious process that is thought to involve some combination of the decay of scalar particles into other matter ("reheating") or some dynamical process where the scalar field coherently pumps energy into other fields ("preheating").

Points 3-5, however, are on quite solid observational footing. We have observations throughout the Universe's history that probe the Universe during these different eras. While we do not fully understand what is driving the Universe today, a perfectly satisfactory model that explains all the observations is that there is a very small constant energy density that has always been pervading the Universe throughout its entire history, but we are only seeing it now because we had to wait until the Universe became dilute enough to see its effects.

  • $\begingroup$ Thank you very much for the in-depth answer! I'll have to follow through and read up more on what you have written but I can follow the general thinking. I believe that Einstein also factored in a 'cosmological constant'. Was that the same thing as they found in the 90's or is it a coincidence of some kind? I mean the accelerating expansion wasn't known to him. But he introduced it to stabilize the Universe. I guess if the constant was a bit higher than he thought then it starts pushing out rather than keeping a perfect balance. $\endgroup$ Jun 25, 2021 at 0:22
  • 4
    $\begingroup$ @RedMarsBlueMoon Right, the cosmological constant has a twisty history. First of all it should have been part of the equations from the beginning, but it was only added when Einstein wanted to prevent the Universe from expanding in his cosmological solutions (but even then the static solution he found is completely unstable to small perturbations in the energy density). After Hubble found the Universe is expanding the cosmological constant was just a theoretical curiosity, until in the 90s the acceleration was discovered. Now the cosmological constant is part of the standard model of cosmology $\endgroup$
    – Andrew
    Jun 25, 2021 at 1:09
  • $\begingroup$ When you say "were not in causal contact with each other before the Big Bang", did you mean to say "were not in causal contact with each other before recombination"? $\endgroup$ Jun 25, 2021 at 11:27
  • $\begingroup$ @DanielHatton No, he got it right. The idea is that the size of space at the time of the Big Bang was significant compared to the speed of light. I.e., the Big Bang is what happened after inflation finished -- it did not happen at a single point but over a huge volume. $\endgroup$ Jun 25, 2021 at 14:14
  • $\begingroup$ @DanielHatton To expand on what Ross said: the "classical" Big Bang model begins with the universe expanding as in point 3, with its size proportional to $t^{1/2}$. That means at the earliest times, parts of space were moving apart too quickly to remain in causal contact for long enough to equalize temperature. By contrast, inflation precedes this phase of radiation-dominated expansion with a period of exponential growth, which can be thought of as happening before the events described by the classical Big Bang. The early stages of this growth were slow enough for temperatures to equalize. $\endgroup$ Jun 25, 2021 at 14:18

The two phases are different. The accelerating expansion immediately after the Big Bang is known as cosmic inflation. Its cause is uncertain, but it is not due to dark energy as the term is usually understood. The other period of accelerating expansion started approximately 5 billion years ago, and this time it is due to dark energy.

It's indeed the case that during the period after cosmic inflation but before the present accelerating expansion, the universe slowed down. This is due to gravity - gravity is attractive and always slows down the rate of expansion of the universe. Dark energy was not "in effect" then because its energy density is small. The energy scale of dark energy is about $10^{-12} GeV$. When the average density of the universe is greater than this value, dark energy does not dominate the universe, and the rate of expansion slows down. The average density of the universe dropped below this value about 5 billion years ago (which makes sense - the universe was always expanding, it was just slowing down, hence the average density drops) and dark energy took over.

  • $\begingroup$ Thank you for this description! It's helping me visualize the sequence of events better. $\endgroup$ Jun 25, 2021 at 0:18

It's a good question and you should be aware of all opinions on this.

Big Bang theory has many successes, but it had problems too, perhaps you could look into the 'horizon problem' and 'flatness problem'.

In order to try and deal with these problems 'inflation' was introduced.

Later on, the 'Big Bang plus inflation' model still had problems. It wasn't matching data from supernovae that seemed to need a currently accelerating universe. So dark energy was introduced.

The Concordance Cosmology is Big Bang plus the seemingly ad-hoc and little understood additions.

It's not understood exactly when or why inflation started and finished and when or why the acceleration due to dark energy began. The cause of each isn't understood.


The best concordance cosmology can do is to make models that involve these quantities. These models show what happened in between, but they are only models with many variable parameters.

Concordance cosmology is able to account for many observations and so is popular with many cosmologists, but it still has problems:

The co-incidence problem, whereby the matter density and cosmological constant density parameter are approximately equal at the current time, also the 'Hubble tension' - different values for Hubble's constants deduced from local measurements and the models.

So, to answer your question, one possibility is that our current cosmological model is wrong and needs improvement.


According to the Standard Model, the rapid expansion of the universe immediately following the big bang (inflationary epoch) lasted for only the tiniest of a moment. This rapid expansion is what allows for the universe to be seen today as being homogeneous (uniform throughout) and isotropic (the same in every direction). It is believed to have occurred due to quantum level effects. These quantum activities lead to the notion of the multi-verse, where many universes, with vastly different properties, have been formed and will continue to form.

After this brief moment our universe returned to a more normal expansion rate which was slowed with time due to gravitational effects. About 5 billion years ago, dark energy became dominant and resulted in an increase in the rate of recession of galaxies as observed by the red-shift of light coming from distant galaxies.

The interesting thing is that when averaged out over the life of the universe, it appears as if the universe has simply been coasting along at a constant rate of expansion. This is determined by the inverse of the Hubble constant. This is an interesting challenge in cosmology because it suggests that we live, right now, at a very special moment in the history of the universe.

There are other ways to look at this by suggesting that the universe is not really stretching, but rather that there is simply more space in the universe as time goes on. https://www.barnesandnoble.com/w/from-falling-apples-to-the-universe-john-r-laubenstein/1139449122?ean=9781649908254

  • $\begingroup$ Thank you for the reply! I like the idea of your book, to explain Gravity for people who don't feel inclined to learn all the math. $\endgroup$ Jun 25, 2021 at 20:45

Your Answer

By clicking “Post Your Answer”, you agree to our terms of service and acknowledge you have read our privacy policy.

Not the answer you're looking for? Browse other questions tagged or ask your own question.