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Crush, rip, freeze or decay – how does the universe end?

The Hubble Space Telescope took a photo of the expanding universe

The universe is expanding – for now.Credit: NASA / ESA

The end of everything (Astrophysical speaking) Katie Mack Scribner (2020)

Scientists know how the world will end. The sun will run out of fuel and enter its red giant phase. Its last eruption of glory will expand and engulf the nearest planets, leaving the earth a charred, lifeless rock. Our planet has about five billion years left.

With this gloomy picture, the theoretical astrophysicist Katie Mack begins her book on the end of the universe ̵

1; a much more uncertain prospect. Cosmologists generally look back, because all the evidence they can examine with a telescope is far away and concerns things that happened a long time ago. Using the motions of distant stars and galaxies to predict possible futures means more speculation.

In Mack’s hands, this speculation creates a fascinating story. Humans are, she writes, “a species that arises between an awareness of our ultimate insignificance and an ability to reach far beyond our everyday lives, into the void, to solve the most fundamental mysteries of the cosmos.” She is a talented communicator of complex physics, and the passion and curiosity about astronomy that has made her a popular speaker and Twitter presence is evident here. (Which are some nerdy jokes and a less convincing code of new physics research that is tangential to the central theme.)

Mack starts at the beginning with the Big Bang. What followed was inflation – a period of rapid expansion. Then structures of dark matter were formed and the building blocks of stars, planets, life and galaxies were assembled. Currently, dark energy, intended to penetrate the universe, somehow counteracts gravity to continue to drive expansion.

The fate of the universe depends on whether that expansion will continue, accelerate or reverse.

The Big Crunch

Astrophysicists have long considered the most likely release to be a reversal of the Big Bang – Big Crunch. Outside our cosmic neighborhood, every galaxy zooms away from us; a clear sign of expansion. If the universe holds enough material, including dark matter, the combined gravitational attraction of all will gradually stop this expansion and precipitate the ultimate collapse. Over time, galaxies, then individual stars, will crush each other more often and kill all life on nearby planets. During the last moments, when densities and temperatures increase in a contracting inferno, all that remains will be extinguished at a single point.

But dark energy can mean another end awaits. The first years of the evolution of the universe were determined by the amount of matter it contained; over the last billions of years, dark energy has begun to dominate and drive the universe outward. Current data from the European Space Agency’s Planck Telescope and other sources indicate that this expansion will continue forever.

Called Heat Death or Big Freeze, this apocalypse will be “slow and annoying,” Mack writes. In thermodynamic terms, she explains, the universe will approach a state of minimum temperature and maximum entropy. As everything gets longer and longer apart, the material from dead stars will spread so that new stars cannot form, and the galaxies they are part of will gradually stop growing. It is like a suffocation of all astrophysical activity, because the fuel for growth and reproduction becomes so diffuse that it is unusable. It is an end “characterized by increasing isolation, unforgivable decay and an eon-long fading into darkness”.

The third downside that Mack discusses is Big Rip. This is in stock if dark energy accelerates expansion even more than currently expected. Like the balloons of the universe eventually, gravitational forces cannot hold galactic clusters together. Stars will be removed from each other, and solar systems like ours do not have the strength to stick together. The remaining stars and planets will explode. Finally, the last atoms will be torn apart.

The latest measurements point to a Heat Death, but a Big Crunch or Big Rip is within their uncertainties.

The last judge scenario that Mack describes is extremely unlikely: vacuum decay. A small bubble of “real vacuum” could form due to instability in the field associated with Higgs boson. This can happen if, for example, a black hole evaporates incorrectly. Such a bubble would expand at the speed of light and destroy everything until it cancels the universe. Vacuum decay may have already begun at some remote location. We will not see it coming.

But not to worry. When Mack rules, whatever it looks like, the end will probably not be close to 200 million years.

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