The Future

Will the universe end?

The Scientific Process: CONCLUSION

“To see a World in a Grain of Sand. And a Heaven in a Wild Flower, Hold Infinity in the palm of your hand, And Eternity in an hour”

William Blake, Auguries of Innocence

WHAT IS THE FUTURE OF THE UNIVERSE?

For most of human history it has been assumed that the universe will go on forever, never changing. The individual stars and galaxies will change, be born, evolve and die, but the overall structure of the universe on a large scale, the scale of the cosmic web, will look the same wherever we look and whenever we look: in the past, in the present and in the future. If we accept the Steady State Theory of the universe then this is the future we will have. The Steady State Theory says that the universe has been here forever and will continue forever.

This is not how the universe will end if we accept the Standard Model of Cosmology (ACDM). In this model the universe does not stay the same but evolves and changes as it cools down, it will look different in the future. How it will look we cannot be sure but we have some theories; quite a few theories. Seven possible futures are discussed and are shown conceptually in Figure 7.1.

1. Big Chill. There are two opposing forces working on the overall structure of the universe: the expansion due to the Big Bang and dark energy, and gravity pulling matter together causing contraction. If the expansion of the universe is pushing the universe apart faster than gravity is pulling the universe together then the universe will carry on expanding forever in what is called the ‘big chill’. As the universe expands it will get colder and galaxies will get further apart until eventually all the matter that exists will be in isolated clusters and galaxies, no longer able to

Figure 7.1: Some of the possible futures of our universe. Credit: C. Dev- ereux and P. Farrell.

merge and form new stars. The cosmic web will be vastly spread out. The universe will be a cold, empty space with isolated lumps of matter.

• 2. Big Rip. W e can take the model further. All objects could be ripped apart by the increasing expansion and the particles will no longer be able to interact. There will be no stars and planets just individual particles floating around in an ever expanding space. This was proposed by Robert Caldwell in 2003 [110] and is called the ‘big rip’ because all matter is ripped apart.
• 3. Big Freeze. We can take the model even further still. Not only could all objects be ripped apart during the expansion but time itself could stop. This is the ‘big freeze’. It can be envisaged that the universe could reach a point where all the particles are spread out uniformly and have reached a maximum amount of disorder (scientists call this entropy). At this point the universe can be considered as being frozen in time. There is movement but nothing changes. Everywhere would look the same and continue to look the same always. Since we measure time by how things change relative to something else, then time can be considered as having stopped. It can be argued that the point that time has stopped is the end of the universe.
• 4. Big Crunch. Instead of the universe forever expanding we could get the opposite. If the amount of matter in the universe has a greater gravitational pull than the expansion has a push, then the universe will eventually stop expanding and start to contract. As it contracts it will become smaller until it goes back to how the universe started, a singularity, all matter and energy at a single point. This is the ‘big crunch’.
• 5. Cyclic Universe. In a future where the universe collapses to a singularity it does not necessarily mean that the universe ends there, it could be the beginning again. The universe resets itself and another Big Bang comes from the singularity and a new universe is born. The process of expanding, collapsing and exploding happens again and again in a continual loop. This ‘cyclic universe’ was proposed by Paul Steinhardt and Neil Turok in 2002 [76] whilst working on string theory. The universe expanding and contracting could last forever or it could eventually collapse into a final singularity.
• 6. Bouncing Universe. The models that I have outlined so far have all required a singularity. The laws of physics break down at singularities, so we don’t like to have them in our models. If we have a model for the universe that allowed us to not have a singularity that would take away a problem. There are some possible universes that do not need singularities.

As far back as 1954, William Bonnor [111] proposed a universe that bounced between an expanding state and a contracting state without any Big Bangs or big crunches. He wanted a universe that is eternal and has no singularities, meaning no Big Bangs. If the universe has enough matter in it then it will stop expanding and start contracting. As it contracts it will heat up again and light could be converted into matter providing an expanding pressure again. The universe then bounces between expanding and contracting like an oscillating wave.

A more recent proposal of the bouncing model came out of string theory by Anna Ijjasa and Paul Steinhardt in 2019 [112]. In this model the universe does not shrink to a singularity but stays a finite size which then becomes the beginning of the new universe. The difference with this model is that the new universe keeps the information on the distribution of matter from the previous universe. They call this model an ‘ekpyrotic universe’ named after the ancient Greek view that the universe is in a cycle of being destroyed by fire and then reborn.

7. Conformal Cyclic Cosmology (CCC). One of the problems with any model that is not infinitely expanding (crunch, cyclic, bouncing) is that they break the second law of thermodynamics. This important law states that everything in the universe slowly becomes more disordered. An example of this is when we break a cup into many pieces it has become more disordered. The process does not work the other way though, the broken cup does not become whole again unless we fix it and then we are creating more disorder elsewhere in the energy we are using to fix it. The problem with a contracting universe is that it is becoming more ordered and so it breaks the second law of thermodynamics. There is also a problem with the Big Bang and the second law. If the universe is become increasingly disordered as it expands then it must have been exceedingly well ordered at the Big Bang. In 2005, the British mathematician Roger Penrose [113] proposed a radical idea for a cyclic universe that he called ‘conformal cyclic cosmology’ (CCC). This model says that the universe is infinitely expanding but that it will reach a point where matter is so spread out that the universe ‘forgets’ how big it is and ‘forgets’ time and it resets itself. The starting point of the new universe now looks the same as the old infinitely expanding universe, the values of the cosmological parameters that define the universe reset themselves and a new beginning happens. This model does not require a singularity and it allows for disorder to be continually increasing so does not break the second law of thermodynamics. It also does not need a very well ordered universe at it’s beginning. It solves three problems with one theory but it is speculative.

How the universe will end is not clear, it is a topic of debate and research and depends on the model of cosmology that is used. The current accepted view of the end of the universe, based on the Standard Model of Cosmology, that the universe will expand forever producing the big chill. However, even in the standard model the big chill prediction is based on measurements on the amount of matter in the universe (dark matter and atomic matter), the rate of expansion of the universe (Hubble’s constant), and the measured value of the acceleration of the expansion (dark energy). These measured values are being refined and we may find that the}' produce a universe that will stop expanding and start collapsing again. It could collapse all the way back until it reaches a single point again, producing the big crunch. There is also the possibility that the universe will expand and contract and then expand again, resulting in a cyclic universe or a bouncing universe that may continue forever. It could be that a modified gravity theory shows that the universe is in a steady state and will continue forever as it is.

So, the answer to how will the universe end is that it could continue forever as it is, either in a steady state or bouncing continually, or it could expand forever to become a cold nothingness, or reset itself to create another universe, or it could contract back to a single point with the possibility of another Big Bang exploding to create a new universe. Predictions into the future are by their very nature speculative but as scientists we like to be able to predict the outcome of our experiments and observations based on the laws of physics. At the moment we don’t have enough evidence to make a good prediction. Although we are not going to be around to find out what does happens to the universe, we would like to have a model that gives us a clear prediction even if we can never test it to it’s final conclusion. To get to a single prediction we need to get more evidence by measuring more accurately the values of mass and expansion in the universe so that we can predict to a high probability what will happen to the universe in the future.

What do we think is the most likely future? The best model we have at the moment is the ACDM Standard Model of Cosmology with well measured values that fit the model. If we take this model and extrapolate it to the future then the values we have measured today tell us that the universe will end with a big chill. It will expand forever with the galaxies getting increasingly further away from each other until eventually we will not be able to see the light from those galaxies. If the galaxies stay together then the Milky Way may still exist although now it will have merged with the Andromeda galaxy and probably other galaxies. Most of the stars will probably be cold and dead with just some small stars giving out the last of their light. What we won’t be able to see is any other galaxies, even with the largest telescopes, because the light from those galaxies will be too far away to get to us. The galaxies will be outside our observable universe. The big chill leaves the universe as a cold, dark place. A chilling prospect, and not one I like the sound of, but it is what the science is telling us.