What Will It Be Like When We Reach The End Of The Universe?

[dufus]

 

The Universe as we know it began some 13.8 billion years ago with the onset of the hot Big Bang. Ever since that early stage, our cosmos has been expanding, cooling, and gravitating in accordance with the laws of physics. As the Universe unfolded, we passed a series of important milestones that led to the Universe we observe and inhabit today. After 13.8 billion years, on one world in an outer arm of a non-descript galaxy on the outskirts of our local supercluster, human beings emerged.

 

It's been spectacular how we've managed to put together our entire cosmic history, from what set up and caused the Big Bang until the present day. But that leads to one spectacular question that humanity has long wondered about: what is our ultimate fate? What will it be like when we reach the end of the Universe? After countless generations of searching, we're closer than ever to the answer.

 

On a local scale, we have our planet orbiting the Sun as one component of our Solar System. But on long timescales, things get exciting relatively quickly. The Sun, as it burns through the nuclear fuel in its core, slowly heats up and becomes more luminous: over the 4.5 billion years that our Solar System has been around, the Sun has increased its energy output by about 20-25%.

 

In another one or two billion years, the Sun's temperature will increase by a great enough amount that Earth will heat up so severely that our planet's oceans will boil. This will effectively end all life on Earth (at least, as we know it) at that time, bringing an end to whatever lives our surviving descendants and our evolutionary cousins continue to enjoy. But the demise of our planet will likely go unnoticed by the cosmos.

 

Sure, there are grander things to think about. As the Universe ages, the rate of star formation continues to plummet. The number of new stars we're forming right now is just a few percent (perhaps 3-5%) of what it was at its peak, some 11 billion years ago. Star formation reached a maximum some ~3 billion years after the Big Bang, and has been falling ever since. To the best of our understanding, most of the stars that will ever exist in the Universe have already been created.

 

And while galaxies will continue to grow by both funneling in new matter from the intergalactic medium and by joining and merging together, most of the structures we're ever going to form have already been formed. Our Local Group of galaxies might all eventually merge together into one giant elliptical galaxy — Milkdromeda, which will primarily form in 4-to-7 billion years when the Milky Way and Andromeda collide — the larger-scale structures aren't really getting any bigger.

 

Yes, the Local Group is relatively small potatoes on a cosmic scale. With two or three (if you include Triangulum) large galaxies alongside perhaps 60 small ones, the Local Group is notable only because it's our home. In reality, groups and clusters of galaxies with dozens, hundreds, or even thousands of times the mass of our Local Group are common across the Universe. The Virgo Cluster, just 50-60 million light-years away, is approximately 1,000 times as massive as our Local Group is.

 

For a long time, we didn't know whether we were gravitationally bound to an even larger structure that included the Virgo Cluster; it was sometimes assumed that we were and it was called the Local Supercluster. Ironically, even though we now have a name for this larger structure — Laniakea — it turns out that there is no such thing as this "supercluster"-scale structure. The reason has to do with the fate of the entire Universe.

 

If you had gone to an astrophysicist in the 1960s, shortly after the Big Bang had been revealed as the source of our cosmic origins, you could have asked them a simple question, "what will the fate of our Universe be?" In the context of the Big Bang and Einstein's General Relativity, there's a simple and straightforward relationship between three things: the expansion rate of the Universe, the total amount and type of stuff inside it, and our fate.

 

You can imagine this as a cosmic race between two players: the initial expansion and the total gravitational effects of everything in the Universe. The Big Bang is the starting gun, and as soon as that gun goes off — as the astrophysicists would have told you — there are three possible outcomes.

 

Recollapse. The expansion starts off fast, but there's enough matter and energy for gravitation to successfully overcome it. The expansion slows, the Universe reaches a maximum size, and recollapses, ending in a Big Crunch.

 

Expansion forever. The expansion starts off fast, and there isn't enough matter and energy to overcome that initial expansion. The expansion rate drops but never reaches zero; the Universe expands forever and ends in a Big Freeze.

 

The "Goldilocks" case. Right on the border between expansion forever and recollapse, this is the critical case. One more proton in the Universe would lead to recollapse, but it isn't there. The expansion asymptotes to zero, but never reverses.

 

For decades, the big quest of the scientific field of cosmology — itself a sub-discipline of astrophysics — was to measure these quantities: how fast the Universe is expanding today and how the expansion rate has been changing over the Universe's history. It's often said, about General Relativity, that "matter tells space how to curve; that curved space tells matter how to move."

 

Well, for the expanding Universe, the expansion tells light how to redshift, and the redshifted light reveals the expansion history of the Universe. Because of the link between spacetime and matter/energy, measuring how the Universe has expanded over its history has the capacity to reveal exactly what the Universe is made of: what the different types of energy in it are and how they compel the Universe to expand.

 

What's remarkable about the past three decades or so is that we've been able to gather enough observations to a high-enough precision that what was once a question for philosophers and theologians — imagining what will happen when we reach the end of the Universe — has now been answered scientifically. Of the three fates we once imagined we now know something remarkable: they're all incorrect. Instead, the Universe surprised us when the answer came in to the questions of what it's made of and what its fate will be.

 

We're not dominated by matter, radiation, or by spatial curvature. Instead, the greatest component of our Universe is dark energy, which will not only cause our Universe to keep expanding, but for the speed of these receding galaxies to increase without limit. Our Universe isn't just expanding, but accelerating: these galaxies will recede faster and faster until they're pushed so far away that we'll never be able to reach them.

 

What does this mean for the fate of our Universe? On the one hand, there are a lot of things we already know. We know that the expansion has been accelerating for some 6 billion years, and that dark energy has dominated the Universe for the entire history of planet Earth. We know that the largest structures that are bound together today — galaxies, galaxy groups, and galaxy clusters — are the largest structures that will ever form; would-be structures on larger scales are being driven apart by this accelerated expansion.

 

And even though everything we see is consistent with dark energy being a cosmological constant, with the same energy density everywhere in space and throughout time, we cannot be certain. Dark energy could still evolve, leading to a Universe that might either recollapse in a Big Crunch, expand forever, or speed up in its acceleration and eventually tear even the fabric of space apart in a catastrophic Big Rip.

 

Right now is a critical time for cosmology, as the coming new generation of space-based and ground-based observatories should help us reveal the answers to these burning questions. Will our Universe continue to expand and accelerate forever? Is dark energy truly a constant in both space and in time? Or does dark energy evolve in some fashion? Is it smooth or inhomogenous? And what, if anything, does that mean for the fate of the Universe?

 

Astrophysicist Dr. Katie Mack, who's making a career out of the attempt to answer this ultimate question (and has a new book coming out on exactly this topic), will be delivering a public lecture in a very special interview-like format this Wednesday, May 6, at 7 PM ET / 4 PM PT, courtesy of the Perimeter Institute. You can watch it, either live or anytime after the lecture is complete, simply by clicking on the embedded video below.

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If dark energy truly is a constant, then we already know how our Universe will end. It will expand forever; the galaxies within groups and clusters will merge together to form a giant super-galaxy; the individual super-galaxies will accelerate away from one another; the stars will all die or get sucked into supermassive black holes; and then the stellar corpses will get ejected while the black holes decay away. It might take googols of years, but eventually, the Universe will be cold, dead, and empty.

 

But this is not the only possibility, as Dr. Katie Mack will help us explore. Join us when the talk occurs in real-time for a live-blog extravaganza (below), or come back any time after it's over to watch the talk in its entirety with the full live-blog presented below. It's your Universe, too. Don't you want to know how the story ends?

 

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[Tickler]

Did we reach the beginning of universe yet or the center?

[dufus]

i,m not here