Curious Kids is a series aimed at children of all ages. If you have a question you’d like an expert to answer, send it to CuriousKidsUS@theconversation.com.
How will the universe end? – Iez M., 9 years old, Rochester, New York
It is not certain whether the universe will “end” or not, but there is every reason to believe that it will continue to be the cosmic home of humanity for a long, long time.
The universe – all of space and time, as well as all of matter and energy – began about 14 billion years ago in a rapid expansion called the Big Bang, but since then it has been in a state of continuous change. First, it was filled with a diffuse gas composed of the particles that make up atoms today: protons, neutrons and electrons. Then this gas collapsed into stars and galaxies.
Our understanding of the future of the universe is informed by the objects and processes we observe today. As an astrophysicist, I observe objects like distant galaxies, which allows me to study how stars and galaxies change over time. In doing so, I develop theories that predict how the universe will change in the future.
Predicting the future by studying the past?
Predicting the future of the universe by extending what we see today is extrapolation. This is risky, because something unexpected could happen.
Interpolation – connecting points within a data set – is much safer. Imagine you have a photo of yourself at 5 years old, then another at 7 years old. Someone could probably guess what you looked like when you were 6 years old. It’s an interpolation.
Using a photo of the author when he was 5 and 7, you could interpolate what he looked like at 6, but you couldn’t predict what he would look like at 29. Stephen DiKerby
Perhaps they could extrapolate from the two images what you would look like when you were 8 or 9, but no one can accurately predict too far into the future. Maybe in a few years you’ll have glasses or suddenly become very tall.
Scientists can predict what the universe will likely look like in a few billion years by extrapolating how stars and galaxies change over time, but things could eventually get weird. The universe and what is in it could change again, as has happened in the past.
How will the stars change in the future?
Good news: the Sun, our medium-sized yellow star, will continue to shine for billions of years. It is about halfway through its 10 billion year lifespan. The lifespan of a star depends on its size. Large, hot, blue stars live shorter lives, while smaller, cooler, red stars live much longer.
Today, some galaxies are still producing new stars, but others have exhausted their star-forming gas. When a galaxy stops forming stars, the blue stars quickly go “supernova” and disappear, exploding after just a few million years. Then, billions of years later, the yellow stars, like the Sun, eject their outer layers into a nebula, leaving only the red stars. Eventually, all galaxies in the universe will stop producing new stars, and the starlight that fills the universe will gradually become reddish and dim.
Billions of years from now – hundreds of times longer than the current age of the universe – these red stars will also disappear into darkness. But until then, many stars will bring light and warmth.
How will galaxies evolve in the future?
Consider building a sandcastle on the beach. Each bucket of sand makes the castle bigger. Galaxies grow over time in the same way by devouring smaller galaxies. These galactic mergers will continue into the future.
In galaxy clusters, hundreds of galaxies fall toward their common center, often resulting in disorderly collisions. In these mergers, spiral galaxies, which are ordered disks, combine chaotically into disordered, drop-shaped star clouds. Think about how easy it is to turn a well-built sandcastle into a disaster by knocking it over.
For this reason, the universe will over time have fewer spiral galaxies and more elliptical galaxies, because spiral galaxies combine into elliptical galaxies.
The Milky Way and the neighboring Andromeda galaxy could thus combine within a few billion years. Don’t worry: the stars in each galaxy would pass each other completely unscathed, and future astronomers would have a fantastic view of the two galaxies merging.
How will the universe itself change in the future?
The Big Bang triggered an expansion that will likely continue into the future. The gravity of everything in the universe – stars, galaxies, gas, dark matter – is moving inward and slowing the expansion, and some theories suggest that the expansion of the universe will continue or slow to a halt.
However, some evidence suggests that an unknown force begins to exert a repulsive force, causing the expansion to accelerate. Scientists call this external force dark energy, but very little is known about it. Like raisins in a cookie, the galaxies will move away from each other faster and faster. If this continues into the future, other galaxies may be too far apart to be observed from the Milky Way.
To summarize today’s best prediction of the future: star formation will stop, so galaxies will be full of old, dark, red stars gradually cooling into darkness. Each group or cluster of galaxies will merge into a single massive elliptical galaxy. The accelerated expansion of the universe will make it impossible to observe other galaxies beyond the local group.
This scenario eventually turns into a dark eternity, lasting billions of years. New data could come to light that changes this story, and the next step in the history of the universe could be something totally different and unexpectedly beautiful. Depending on how you look at it, the universe may not have an “end” after all. Even though what exists is very different from the current universe, it is difficult to imagine a distant future where the universe has completely disappeared.
How do you feel in this scenario? It makes me nostalgic sometimes, which is a kind of sadness, but then I remember that we live in a very exciting time in the history of the universe: right from the beginning, in an age full of exciting stars and galaxies to observe! The cosmos can sustain human society and curiosity for billions of years into the future, so we have plenty of time to continue exploring and searching for answers.
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This article is republished from The Conversation, an independent, nonprofit news organization that brings you trusted facts and analysis to help you make sense of our complex world. It was written by: Stephen DiKerby, Michigan State University
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Stephen DiKerby receives funding from the National Science Foundation.
