In my last blog post I wrote about how astronomers have accumulated overwhelming evidence that the universe will continue to expand at an ever increasing rate. This accelerating cosmic expansion rate is due to slightly more than two-thirds of the universe being composed of dark energy.
Since the posting of my last article, many of you have asked, what does an ever-increasing cosmic expansion rate mean for the future of the universe and for us human beings in particular? If by the future, you mean the next several thousand years, the answer is nothing of any significance.
First Consequence of Cosmic Acceleration
The first major consequence of dark energy accelerating the cosmic expansion rate is that “soon” we will not be able to observe back to the beginning of the universe. Because of light’s finite velocity, the farther away astronomers observe, the farther back in cosmic history they see. The universe is now old enough that dark energy is causing those regions of space associated with the cosmic creation event to move away from us at just a hair under the velocity of light. In the not too distant future, dark energy will cause those regions of space to move away relative to us at greater than light’s velocity. When that happens, the cosmic creation event will cease to be visible.
Owing to measuring uncertainties, astronomers cannot yet put an accurate date on when in the future the cosmic creation event will cease to be observable. It could be less than a few hundred thousand years hence. It could be many millions of years. It certainly is much less than a billion years. What is certain is that from now on progressively less and less of the universe will be observable (a depressing thought for us astronomers to contemplate).
An ever-accelerating cosmic expansion rate has several much more dire consequences for the distant future. I wrote about some of these consequences in my book Why the Universe Is the Way It Is.1
Most Dire Consequence
The most dire possible consequence of accelerating cosmic expansion is something astronomers call the Big Rip. The Big Rip, also known as phantom energy, is a hypothesis about the ultimate fate of the universe that will occur if a certain property of dark energy proves to be true. That critical factor is the value of the equation of state for dark energy.
The equation of state for dark energy, ω, is the ratio of dark energy pressure to dark energy density. If ω <-1, then the Big Rip will occur. In 2003, three theoretical physicists, Robert Caldwell, Marc Kamionkowski, and Nevin Weinberg, published a paper in Physical Review Letters where they determined the “cosmic doomsday” consequences of the Big Rip.2
As already noted, a universe expanding at an ever-increasing rate eventually results in a shrinking fraction of the universe that can be observed. When a particular structure within the universe becomes larger in size than the observable universe, none of the four fundamental forces of physics (electromagnetism, weak nuclear, strong nuclear, and gravity) can operate between the most distantly separated parts of the structure. When such operations become impossible the structure is ripped apart.
Where ω <-1, the first cosmic structures to be ripped apart from one another will be clusters of galaxies. Later in the history of the universe, galaxies will be ripped apart from one another. Later still, the fundamental forces of physics will be unable to hold our Milky Way Galaxy together. It will be torn apart. Later yet, the same fate will befall our solar system. After that, stars, planets, asteroids, and comets will be torn apart. In an instant before the end, all molecules and atoms will be destroyed. At the very end, space and time are ripped apart and the universe ceases to exist.
In their paper, Caldwell, Kamionkowski, and Weinberg calculated dates for when all these ripping apart events will occur for a hypothesized value of ω = -1.5. In this example the universe is utterly destroyed 22 billion years from now. The Milky Way gets ripped apart about 60 million years before the end. The solar system gets ripped apart about three months before the end. A few minutes before the end, stars, planets, asteroids, and comets cease to exist. A second before the end, molecules and atoms are destroyed.
No Need to Worry
Before you get overly concerned about the future of your investment portfolio, let me assure you that we astronomers possess measurements that tell us the value of ω is larger than -1.5. As the value of ω changes upward from -1.5 toward -1.0, the amount of time that passes before the Big Rip occurs increases exponentially. If the value of ω exactly = -1.0, the Big Rip occurs at an infinite time in the future. If the value of ω is greater than -1.0, the Big Rip becomes impossible.
In the November 20, 2016 issue of the Astrophysical Journal a team of 77 astronomers published cosmological constraints they had derived from the largest survey to date of clusters of galaxies.3 They significantly improved the precision of the measure of ω. Accordingly, ω = -1.023±0.042. Another team, consisting of 13 astronomers, published a paper in which they reported on the expected results from the European Space Agency’s Euclid satellite.4 This satellite will yield an even bigger and deeper survey of galaxy clusters. This survey will produce a measure of ω with an error bar of only ±0.02.
Already, the best measured value of ω is consistent with ω = -1.0. If it is exactly -1.0, there will be no Big Rip in finite time. If it is only very slightly less than -1.0, the Big Rip will occur in the extremely distant future, likely later than the half-life for proton decay. Half of all the protons and neutrons in the universe will have decayed away by the time the universe is a billion trillion trillion years old.
From a Christian perspective the Big Rip, even if it were to happen, has no impact on life. The Bible teaches a two-creation model. Once God has conquered and permanently eradicated evil, the universe will have fulfilled its purpose. Then, God will replace the universe with the new creation that he describes in the last two chapters of the Bible. It is in the new creation, not the universe, that humans who so choose will spend eternity with God and one another. The Bible promises in multiple passages that God has already conquered evil, and he will permanently eradicate evil long before any possible Big Rip can even begin to occur.
Robert Caldwell, Marc Kamionkowski, and Nevin Weinberg, “Phantom Energy: Dark Energy with ω <-1 Causes a Cosmic Doomsday,” Physical Review Letters 91 (August 2003): id. 071301, doi:10.1103/PhysRevLett.91.071301.
Tijmen de Haan et al., “Cosmological Constraints from Galaxy Clusters in the 2500 Square-Degree SPT-SZ Survey,” Astrophysical Journal 832 (November 2016): id. 95, doi:10.3847/0004-637X/832/1/95.
Barbara Sartoris et al., “Next Generation Cosmology: Constraints from the Euclid Galaxy Cluster Survey,” Monthly Notices of the Royal Astronomical Society 459 (March 2016): 1764–80, doi:10.1093/mnras/stw630.
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