The universe is not only expanding, but accelerating that expansion, leading most scientists to anticipate it will keep on growing for a very long time, if not forever. However, a trio of Princeton physicists have challenged this view, presenting a model of the universe in which this expansion is nearly at its end. The universe will start to contract in on itself, they claim, and that could happen surprisingly soon. This is a cosmologist’s “soon”, however, of the order of 100 million years, not something most people would recognize as imminent.
The discovery of acceleration in the expansion of the universe has shaken up cosmology perhaps more than anything else this century. Beforehand the primary debate was whether the universe would expand forever, albeit more slowly, or be dragged back into a “big crunch” as gravity overcame the movement apart.
Acceleration, and the Dark Energy used to explain it, appeared to end the possibility the universe would ever contract again, but a minority of physicists aren't ready to let the idea go. Professor Paul Steinhardt, in particular, has proposed “bouncing” models of the universe. Now Steinhardt and co-authors claim in Proceedings of the National Academy of Sciences that the turning point from expansion to contraction could be close without us being able to tell.
The authors do not assert certainty. They refer to three models of Dark Energy's nature. One of these would see the universe continue to expand faster and faster forever, while a second would see it slow at an unpredictable point, probably far in the future.
However, the paper asks us to consider a third model, where Dark Energy is a type of quintessence and the rate of acceleration is slowing down. The universe, in this scenario, is like a car whose driver took off from the lights with all the power they could muster, but is now easing their foot off the accelerator, without having fully removed it yet. Consequently, expansion is happening at the fastest rate it ever has, but the acceleration is small compared to earlier times.
Eventually, in this scenario, acceleration will cease. After a point where the universe's expansion is stable, it will start to slow down, before eventually everything stops expanding altogether and starts to contract.
Such models have been proposed before, particularly by Steinhardt. What is new here is an effort to estimate how close we could be to the two crucial points, first where acceleration ceases, and then when contraction begins. Under what the authors call the Quintesscence-Driven Slow Contraction CDM Model (QDSCCDM) they calculate the end of expansion could be less than 100 million years away, less than one percent of the age of the universe. The space between the end of acceleration and the beginning of contraction might be of similar length.
Even if the QDSCCDM is right, this is a lower bound – the timescales depend on factors beyond our current technology to measure, so these events could be much further away. If the model presented is considered plausible, however, it might spark projects to develop our capacities to the point where we can measure whether the turning point is near.
Steinhardt might fairly be described as recently voyaging outside the cosmological mainstream. In 2017, a letter he co-authored criticizing the widespread acceptance of an “inflationary” period immediately after the Big Bang attracted a scathing response from luminaries including Stephen Hawking. On the other hand, he is also not someone who can be lightly dismissed. Not only did he help lay the foundations for inflationary cosmology himself, his work on the effect of gravitational waves on the cosmic background radiation and the more recent discovery of quasicrystals have earned him immense respect in the field.