All good things must come to an end.
And one day the universe will, too. It won't happen for a long time, but eventually the galaxies will wink out, and the universe as we know it will be a thing of the past.
This has been confirmed by the most comprehensive assessment ever conducted of the energy output of the nearby universe. An international team of researchers examined the energy output of over 200,000 galaxies, and determined that they are currently producing only half the amount of energy they were producing 2 billion years ago.
The study, part of the Galaxy and Mass Assembly survey, made use of data collected from seven telescopes, including NASA's GALEX and WISE orbiting telescopes, the European Space Agency's Herschel telescope, the European Southern Observatory's VISTA and VST telescopes and the Anglo-Australian Telescope in Australia. It was led by Simon Driver, a research professor at the International Centre for Radio Astronomy Research in Western Australia.
"We used as many space and ground-based telescopes we could get our hands on, to measure the energy output of over 200,000 galaxies across as broad a wavelength range as possible," Driver explained in a statement.
The aim of the research was to map all the energy generated within a particular volume of space. Each of the galaxies was measured at 21 wavelengths, which confirmed in detail what has been known since the 1990s: The universe's life as we know it is finite.
This is not exactly unexpected. All energy in the universe was produced during the Big Bang. For around 3 billion years, the universe (currently around 13.8 billion years old) was exploding with the formation of stars and galaxies, after which it slowed down. It has been slowing down ever since.
"While most of the energy sloshing around was created in the aftermath of the Big Bang, additional energy is constantly being released by stars as they fuse elements like hydrogen and helium together," Driver said.
"This newly released energy is either absorbed by dust as it travels through the host galaxy, or escapes into intergalactic space and travels until it hits something such as another star, planet, or very occasionally a telescope mirror."
Sarah Sweet, a postdoctoral research fellow with the Research School of Astronomy and Astrophysics, who did not participate in the survey, explained in an email that the results definitely show the universe slowing down.
"The GAMA team found that the total amount of energy produced by the universe today is about 1.5 times less than it was 2 billion years ago," she said.
"We know that most of this energy is in the form of starlight and glowing dust, so their finding indicates that fewer new stars are being formed and more old stars are dying today than in the past. These stars can be thought of like cells in a body: The universe's 'cell regeneration' is failing, so we can think of the universe as 'dying.'"
The expansion of the universe is also accelerating, which means that over long periods of time, objects will move farther and farther apart from each other. Current theories indicate that the universe will grow cooler and darker and, if there are any humans left at that stage, other galaxies will move too far away to be seen. This is all explained by University of Western Australia associate professor and research fellow Aaron Robotham, who took part in the research, on The Conversation. But, he notes, this process will take a very long time.
"After trillions of years we will only be able to see our own galaxy as the others will have raced too far away. After hundreds of trillions of years no new stars will be made anywhere at all," he said.
"Next our galaxy will eject most of its remaining stars into the cosmic void, and what is left will collapse into our central black hole. All matter as we know it will eventually decay, the black holes will evaporate and what is left will be a very lonely and empty place."
The team plans to expand their research with the help of the Square Kilometre Array, the world's largest radio telescope, which is currently under construction.
"The Universe is fated to decline from here on in, like an old age that lasts forever. The Universe has basically plonked itself down on the sofa, pulled up a blanket and is about to nod off for an eternal doze," Driver said.
Driver presented his research paper, "Galaxy And Mass Assembly (GAMA): Panchromatic Data Release (far-UV--far-IR) and the low-z energy budget," at the International Astronomical Union in Honolulu, Hawaii, on August 10. The paper has also been submitted to the Monthly Notices of the Royal Astronomical Society.