By Don Lincoln, Fermilab
Astronomers now believe that matter itself is only a small part of the matter and energy budget of the universe. Scientists claim that the majority of matter and energy in the universe is called dark energy. But, what is that?
Dark Energy in the Universe
The Supernova Cosmology Project (SCP) observed that about five billion years ago (just about the time our sun began to coalesce out of interstellar gas) the expansion rate of the universe changed from slowing down to speeding up. It appears that five billion years ago, some sort of repulsive form of gravity came into existence. While the jury is still out on what this repulsive gravity is, it has a name and we call it dark energy.
In this moment when astronomers are beginning to look at the sky, ordinary matter makes up about 5% of the matter and energy of the universe. Dark matter makes up about 25% of the universe. And dark energy makes up a whopping 70% of the universe that is more than double the amount of ordinary matter and dark matter combined.
Dark Energy Versus Dark Matter
Even though the term ‘dark energy’ sounds a lot like dark matter, the two are totally different. Dark matter is a form of matter that doesn’t emit light but responds to gravity in the same way as ordinary matter.
Dark energy isn’t matter at all. It’s some sort of energy field that not only counteracts, but can even overpower, the attractive force of familiar gravity.
Dark Energy Versus Cosmological Constant
The two terms ‘dark energy’ and the ‘cosmological constant’ might seem to be same, but in principle they are not. The term ‘dark energy’ is a phrase coined for whatever is the cause of the accelerating expansion of the universe. It’s simply a vague term for an unexplained phenomenon.
The term ‘cosmological constant’ is a specific proposed model for dark energy. It’s a mathematical construct. Like the name suggests, it is cosmological in nature. It permeates the universe and it controls its future. It is also a constant density of energy.
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Models of Dark Energy
Since there is not enough data to prove that the cosmological constant actually is dark energy, one should think of the cosmological constant as a possible model of dark energy. Perhaps the defining feature of the cosmological constant is that it is a constant energy density, with the emphasis on constant. There are other models of dark energy that aren’t constant. The generic term for models of dark energy that change in time is called quintessence.
The first version of quintessence was proposed in 1988 by two physicists, Indian-born Bharat Ratra and Canadian-American Jim Peebles. The specific name of quintessence was proposed in 1998. Its name stems from the Latin quinta essentia, which means ‘fifth element’.
Is Dark Energy Constant?
The data from the SCP and High-Z experiments were consistent with dark energy being constant, as are data from more recent studies, but these kinds of measurements are notoriously difficult. While scientists say that the measurements are consistent with a constant dark energy, there is some wiggle room. It could be that dark energy is changing over time.
Even if dark energy were pretty constant in the past, there is no guarantee that it will remain constant in the future. After all, something made dark energy become relevant about five billion years ago. If there was a change then, maybe there will be a change in the future.
What if Dark Energy Was Really Constant?
The universe came into existence somehow and started expanding. Inflation turned on and turned off. As the universe expanded and cooled off, the volume increased and the total amount of dark energy increased, while the total amount of matter of all kinds stayed the same. Or, the density of all matter decreased, and the density of dark energy stayed the same.
And, about five billion years ago, the total amount of dark energy eventually equaled the total amount of matter. Before that time, the universe was dominated by the attractive gravitational force due to matter and, after that time, the universe was governed by the repulsive form of dark energy. And, in these modern times, dark energy is continuously pushing the universe apart.
So, the density of dark and ordinary matter will continue to drop, while dark energy will remain constant. Dark energy will expand space faster and faster, pushing distant galaxies away from us. Eventually, they will be so far away and moving so fast that light from them will never reach us.
What if Dark Energy Decreases or Increases with Time?
If quintessence is real and if dark energy gets smaller with time, then dark energy won’t be as much to push the universe apart like it does today. Maybe, in the distant future, we’d still be able to see some distant galaxies under that scenario.
If dark energy increases over time, it will cause the expansion to accelerate even more. The distant galaxies would disappear, but also even closer ones. It could be that at that distant time, we’d only see the dying stars of the Milky Way and Andromeda, after they collide in a few billion years.
If dark energy increases even more, the expansion of space could rip apart Milkomeda. More energy, and it would rip apart the solar system, or Earth, or all of matter. If dark energy increases enough, the future of the cosmos will be individual atoms floating isolated in a huge and empty cosmos, with all of the other atoms ripped away by the dark energy driven expansion of space.
Common Questions about Dark Energy
Scientists claim that the majority of matter and energy in the universe is called dark energy. it is a kind of repulsive form of gravity that accelerated the expansion of the universe.
The first version of quintessence was proposed in 1988 by two physicists: Bharat Ratra and Jim Peebles. The specific name of quintessence was proposed in 1998. Its name stems from the Latin quinta essentia, which means ‘fifth element’. The generic term for models of dark energy that change in time is called quintessence.
If dark energy increases over time, it will cause the expansion to accelerate even more. Not only the distant galaxies would disappear, but also even closer ones.