How Will the Universe End? It All Depends on Dark Energy.

 25 years since scientists discovered dark energy, we still have no clue what it exactly is. But as we learn more, it becomes clear that dark energy pretty much defines the future of the universe. 

In the past five billion years, the expansion of the universe has accelerated; Source: Wikipedia

The questions that bother astrophysicists for more than half a century: what is dark energy? Is it really dark? And why is it so difficult to find?

All of these questions are easy and hard to answer at the same time. We know that dark energy drives the accelerated expansion of the universe that started about five billion years ago, around the same time the Sun was born. 

Based on our theories, dark energy makes up 68% of the entire universe — so it’s really important that we understand what it is. 

We may not have an idea what dark energy is exactly, but we have pretty sophisticated theories on how it affects the evolution of the universe. 

If you want to learn more about the future of the universe and how galaxies will evolve, check out this article. 

Our theories postulate that the expansion of the universe accelerated around five billion years ago, right as the gas and dust in a stellar nursery in the Milky Way collapsed to form our beloved Sun. And it seems that this expansion won’t stop very soon — in fact, it might continue forever. 

As the universe expands infinitely, distant galaxies will become so redshifted that their light is no longer able to reach any observers. Meanwhile, gravitationally bound galaxies (such as those in the Local Group) will merge into one gigantic elliptical galaxy. 

In the far future, when there’s no gas available to form new stars, the universe will have died a heat death, now popularily called the Big Freeze. 

(It has been theorized that at this point, when the universe is nothing but an empty shell of its former self, the fabric of spacetime will literally be ripped apart, a delayed version of the Big Rip theory.) 

Thanks dark energy for such a solemn end! However, though the theory is the most widespread one, if we learn more about dark energy, the outcome could be drastically different. 

And new research indicates that there’s a lot we’re wrong about in regards to dark energy. 

Over the course of five years, astronomers observed distant supernovae all across the universe, seeking one particular type of supernova: type Ia. Such supernovae are very distinct and occur only in binary systems with a red giant and a white dwarf. 

Here, the white dwarf accretes matter from the red giant until it reaches a maximum mass of 1.4 solar masses. When the white dwarf reaches this mass, there’s no turning back anymore; it’ll explode in a supernova. 

The supernova remnant of probably one of the most famous supernovae in history: Kepler's Supernova. It was a Type Ia supernova. Source: Wikipedia

But what do type Ia supernovae tell us about the universe? 

Because they are very reliable and constant in their energy output, they’re good standard candles. And finding type Ia supernovae at different redshifts can tell us a lot about the expansion of the universe. 

In the Dark Energy Survey, scientists found a total of 1’500 type Ia supernovae, some as far as six billion light years away. 

The observations show that dark energy has indeed accelerated the expansion of the universe — and that as space expands, there will be more dark energy. Does this mean that dark energy isn’t a product of space, but instead, a fundamental part of space itself? 

At least that’s what the data indicates. 

What astronomers also learned was that dark energy isn’t constant, as was previously assumed, but that it’s changing, getting stronger sometimes and then weakening again. 

That observation was backed up when another survey uncovered similar results. 

Since 2019, the Dark Energy Spectroscopic Instrument (DESI) experiment has mapped galaxies and their redshifts using the Mayall Telescope on Kitt Peak, Arizona to learn more about dark energy. DESI aims to map 37 million galaxies by the end of its survey that will last five years. 

Not long ago, we received the results from the first year of observations — and, to put it bluntly, they will redefine our entire knowledge about dark energy. 

According to the first-year data from DESI, dark energy does indeed drive the expansion of the universe, but it’s been getting weaker. 

If that is true, and that has to be confirmed by more data, then it means that the expansion of the universe is variable. 

It could paint a much more brutal end for the universe. 

What the Big Crunch has what the Big Freeze hasn’t is gravity. In the Big Crunch scenario, gravity will overwhelm the expansion of the universe and pretty much overturn the Big Bang. All matter and the universe itself would fall in on itself, essentially forming the most massive black hole the universe (or multiverse?) has ever seen. 

If a Big Crunch happens, all matter, including stars and galaxies, would essentially be cooked as space itself reaches incredibly hot temperatures. It then all falls in on a dense singularity. 

Okay, great. The universe will either be too cold for anything to sustain itself, or too hot for anything to exist. 

But this doesn’t explain what dark energy is. 

While there is a conclusion to the universe itself, there’s no satisfying answer to explain what dark energy is. 

It might be a scalar field (quintessence), an unknown particle, maybe something hidden in an unreachable dimension (string theory) — or it might be more fundamentally an energy of space itself. And the evidence for this could lie in supermassive black holes (SMBH). 

When we look at the young universe, SMBH often seem… too massive. We already know that our traditional theories about how SMBH form can’t be right, but one idea is that black holes get their mass from dark energy in the form of vacuum energy. 

It would be the closest thing that would describe the cosmological constant — something that Einstein himself thought was wrong. 

In the end, since we can’t know with certainty, our best deal with dark energy is to relentlessly hunt it down, and find the answers that we are looking for in the deepest regions of space. 

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