Aug 25 / Aman Burman

The fate of the universe

Introduction: The ultimate fate of the universe is a subject of study in the field of cosmology. Cosmology deals with the essential questions of the cosmos: When did the universe begin? How did it start? Has the universe always been expanding? However, the question which scientists have yet to figure out the answer to is how is the universe going to end?

For decades, scientists had accepted that distant galaxies are all moving away from us, with the further ones moving away from us the fastest. When a galaxy moves away from us, the light that it emits is shifted towards the red end of the spectrum, as its wavelength get longer. If a galaxy moves closer, the light moves to the blue end of the spectrum, as its wavelength gets shorter. Edwin Hubble in 1929, showed that nearly all galaxies he observed are moving away from us. He also concluded that the further a galaxy is away from us, the faster it is moving away. Thus, the only reason for these observations is if the universe is expanding.

In the early 1990s, people were certain that gravity was going to slow the expansion of the universe as time went on. The slowing had not been observed yet, but theoretically, the universe had to slow. The universe is full of matter and the attractive force of gravity pulls all the matter together. However, in 1998, the Hubble Space Telescope (HST) had made observations of very distant supernovae that showed that, a long time ago, the universe was expanding more slowly than it is today. This went against the theories of most scientists who had thought that the expansion of the universe would gradually start slowing down when in fact, the expansion has been accelerating. No one understood why this phenomenon was occurring but something present in the universe was causing it and scientists called it dark energy.

More is unknown than is known about dark energy. We know how much dark energy there is because we know how it affects the universe's expansion. Other than that, it is a complete mystery.  Roughly 68% of the universe is dark energy. Dark matter makes up about 27%. The rest adds up to less than 5% of the universe.

One explanation for dark energy is that it is the cosmological constant that Albert Einstein had theorised in 1917. In this notion, dark energy is a property of space itself meaning it will not dissipate as the universe becomes larger. As more space comes into existence, more of this energy would appear. As a result, this form of energy would cause the universe to expand faster. Unfortunately, no one understands why the cosmological constant should even be there, much less why it would have exactly the right value to cause the observed acceleration of the universe. Another explanation for dark energy is what some theorists have name as quintessence. Quintessence is an exotic kind of energy field that pushes particles away from each other, overpowering gravity and the other fundamental forces.

The thing that is needed to decide between dark energy possibilities - whether it is a property of space, a new dynamic fluid, or a new theory of gravity - is more and better data.

The Big Bang marks the starting gun of the greatest race of all time: between gravity and the expansion rate of the universe. Which one will eventually win in our Universe? The answer to that question should determine our Universe's fate. The three main proposed fates of the universe are the big rip, the big freeze and the big crunch.

The Big Rip

One of the biggest surprises in all of physics came at the end of the 20th century: in 1998. By looking at some of the most distant events arising from a single star — type Ia supernovae — we were able to determine that the Universe wasn't just expanding, but accelerating. There must be something more than just matter, radiation, and the curvature of space filling the Universe.

There needed to be a new form of energy that caused distant galaxies to accelerate away from us. This mysterious dark energy might be a cosmological constant, but it might be something more interesting. One interesting option could change the fate of the Universe, resulting in a Big Rip.

We know this from how the Universe has expanded over its history. About 6 billion years ago, distant galaxies started to speed up in their recession away from us: the Universe was accelerating. Based on our observations of how they're moving now, as well as observations of the cosmic microwave background, large-scale structure, and other indicators, we've determined that the Universe is 68% composed of dark energy.

This energy doesn't appear to drop in density as the Universe expands, unlike matter and radiation. Whereas matter becomes less dense as the volume of the Universe increases, and radiation also redshifts to longer, less energetic wavelengths, dark energy is an energy inherent to space itself. As new space is created, the energy density remains constant.

This allows us, in theory, to predict the fate of the Universe. If dark energy were truly a constant type of energy that didn't change as the Universe expanded, the Universe would simply expand forever. The Hubble rate of expansion would asymptote to a constant, finite value of approximately 55 km/s/Mpc.

Unlike in a scenario where there wasn't any dark energy, the Universe is accelerating. As far as our observations indicate, it will continue to accelerate at this constant rate arbitrarily far into the future. The fate of the Universe should be cold, empty, and lonely; only the objects that are already gravitationally bound today will remain within reach of one another.

This assumes, however, that dark energy is truly a cosmological constant. It assumes that it doesn't increase or decrease in strength or density, that it doesn't change sign, and that it doesn't vary across space. We have good evidence that this is the case, mostly from observations of how galaxies cluster on the largest scales.

But even with the best observations that we have, we cannot be certain that dark energy is a cosmological constant. It could vary with time somewhat substantially, increasing or decreasing by no more than a certain amount. The way we quantify how much dark energy can vary is with a parameter called w, where if w = -1 exactly, it's a cosmological constant. But observationally, w = -1.00 ± 0.08 or so. We have every reason to believe its value is -1, exactly.

If dark energy isn't a constant, there are two major possibilities for how it could change. If w becomes more positive over time, then dark energy will lose strength, and potentially even reverse its sign. If this is the case, the Universe will stop accelerating and the expansion rate will drop to zero. If its sign reverses, the Universe may even recollapse, fated for a Big Crunch.

There is no good evidence that indicates this will be the case, but next-generation telescopes like the LSST, WFIRST, and EUCLID should be able to measure w down to an accuracy of 1-2%, a vast improvement over what we presently have. These observatories should all come online in the 2020s, with EUCLID scheduled to get there first: launching in 2021.

Of course, there's always the option that w becomes more negative over time, dipping below -1 and remaining there. If this is the case, something fascinating happens: the Universe experiences a fate known as the Big Rip.

If dark energy is truly constant, than things like our Solar System, our galaxy, and even our local group of galaxies — consisting of the Milky Way, Andromeda, the Triangulum Galaxy, the Magellanic Clouds and a few dozen small, dwarf galaxies — will remain gravitationally bound together for trillions upon trillions of years into the future.

But if dark energy is increasing in strength, which it will do if w < -1, then that acceleration rate will not only drive distant galaxies away from us, but will cause these large-scale structures to become gravitationally unbound as time goes on!

Dark energy will get stronger and stronger over time, and this will have dire consequences for the fate of everything that makes up our Universe today.

When the energy density of dark energy increases to about ten times what it is today, it will be enough to prevent the Milky Way from merging with Andromeda. Instead, this "Big Rip" scenario will drive our neighboring galaxy away from us, like all the other distant galaxies in the Universe. Also gone would be the Triangulum Galaxy and most of the other dwarf galaxies as well. But this won't be the end; dark energy will continue to increase in strength.

Increase the energy density of dark energy to about a hundred times its current value, and the stars on the Milky Way’s outskirts will  begin flying off from our galaxy. The metric expansion of space will be able to overcome even the gravitational pull of all the matter in our local neighborhood, both normal and dark.

And if you increase dark energy's strength to two or three hundred times its present value, our Sun will join those outer stars in being torn apart from our galaxy. Our Solar System will fly through intergalactic space all by its lonesome.

But this won't be the end of what we'll lose in a Big Rip scenario. The energy density of dark energy will continue to rise, and eventually, this will threaten the existence of not just our Solar System, but every Solar System in the Universe. When dark energy becomes strong enough, the planets themselves will become unbound from our Sun.

The Oort cloud will go first, followed by the scattered disk and the Kuiper belt, and then Neptune, Uranus, Saturn, and Jupiter. The asteroids would go next, followed by Mars. Earth will get thrown out of orbit when dark energy reaches a density that's 100 billion times its present value.

And finally, here on Earth, the ultimate catastrophe would befall anyone left behind. Humans would be separated from Earth’s gravitational pull, individual cells, molecules, atoms, and nuclei would be torn apart, as the dark energy density continued to increase to an infinite amount. Presumably, even the fundamental fabric of spacetime itself would be torn apart at the very end.

Thankfully, with the constraints we have on dark energy today, that w = -1.00 ± 0.08, we have time. If the Universe will end in a Big Rip, that's a fate that won't befall us until 80 billion years from now at the earliest: nearly six times the present age of the Universe. The unbinding of galaxies from one another, the very first notable step on the path to a Big Rip, won't occur for many tens of billions of years even in the most pessimistic viable scenario.

To the best of our knowledge, there isn't any robust data that favors dark energy increasing in strength versus remaining constant, but we have to get more sensitive to know for sure. What is certain, however, is that no matter what the evidence indicates, we'll measure it better than ever before as the 2020s unfolds, with the Earth, Sun, galaxy, and local group all safe from this fate for many generations of stars to come. The Big Rip, although not ruled out, at least lies a very long time in the future.

The Big Freeze

The second fate of the universe is the big freeze Also somewhat conversely called 'Heat Death', this scenario is believed to be the most likely according to what we already know about physics and the Universe. Thermodynamics is the study of heat and energy and how they influence each other. The first law of thermodynamics states that energy cannot be created or destroyed, only transferred into different forms. The fact that energy cannot be created suggests that heat and energy in the universe is in fact finite – meaning as the universe expands, it will cool, as it has been doing.

The laws of thermodynamics have led experts to the possible conclusion that the universe will end in a ‘heat death’. The Heat Death suggests the universe will continue to expand. However, rather than ripping apart like The Big Rip theory, it will expand until it is nothing. The second law of thermodynamics states that entropy (a measure of uncertainty) increases in an isolated system. This suggests that as the universe becomes bigger, matter and energy will become evenly spread throughout the universe until its temperature begins to decline as it reaches absolute zero which is –273.15 degrees Celsius. Mechanical motion caused by energy and heat being distributed within the Universe will cease. During this Big Freeze, the Universe would, in theory, become so vast that supplies of gas would be spread so thin that no new stars can form. Under that model, time becomes an endless void in which nothing ever happens as there is little to no energy left in the Universe.

The big crunch

Big Crunch hypothesis is a symmetric view of the ultimate fate of the universe. Just as the Big Bang started as a cosmological expansion, this theory assumes that the average density of the universe will be enough to stop its expansion and the universe will begin contracting. The end result is unknown; a simple estimation would have all the matter and space-time in the universe collapse into a dimensionless singularity back into how the universe started with the Big Bang, but at these scales unknown quantum effects need to be considered (see Quantum gravity). Recent evidence suggests that this scenario is unlikely but has not been ruled out, as measurements have been available only over a short period of time, relatively speaking, and could reverse in the future.  The big crunch states that the expansion of the universe will stop accelerating and eventually slow down.  If dark energy becomes weak enough, gravity might ultimately win the tug of war and pull the universe back onto itself. This would result in The Big Crunch. This would ultimately cause the Universe to shrink and become compressed. Stars, planets and entire galaxies would clump closely with each other causing the universe to collapse in on itself.

Models of a collapsing universe of this kind suggest that, at first, the rate of contraction would be slow, but the pace would gradually pick up. As the temperature begins to increase exponentially, stars would vaporize and eventually atoms and even nuclei would break apart, completely opposite to the early stages after the Big Bang. In a Big Crunch, dark energy would weaken and reverse sign, causing the Universe to reach a maximum size, turn around, and contract. It could even give rise to a cyclical Universe, where the "crunch" gives rise to another Big Bang. If dark energy continues to strengthen, however, the opposite fate occurs, where bound structures eventually get torn apart by the increasing expansion rate.

The scenario that will lead to the end of the universe depends on many factors. This includes the exact shape of the universe, the amount of dark energy it holds and changes in its expansion rate. As of now, the most likely end to our universe will be the big freeze but the good news is we have trillions of years before any of this takes place.

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