What Does The Expansion Of The Universe Tell Us About The Future?

We know the speed of light is constant, but what about the speed at which the universe is expanding?
Astronomers think that the expansion of the universe is regulated by both the force of gravity, which acts to slow it down, and a mysterious dark energy, which pushes matter and space apart. In fact, dark energy is thought to be pushing the cosmos apart at faster and faster speeds, causing our universe’s expansion to accelerate. Image credit: NASA/JPL-Caltech

Astronomers think that the expansion of the universe is regulated by both the force of gravity, which acts to slow it down, and a mysterious dark energy, which pushes matter and space apart. In fact, dark energy is thought to be pushing the cosmos apart at faster and faster speeds, causing our universe’s expansion to accelerate. Image credit: NASA/JPL-Caltech

Originally posted on Forbes!

Ah yes. The speed at which the Universe is expanding is not a constant. This is a very interesting consequence of the presence of dark energy in our Universe, and gives us an interesting view into the very distant future of our Universe. 

Dark energy is a descriptive term we have applied to a force which is responsible for the observed expansion of the Universe.  We can see that the Universe is expanding, by measuring the apparent speeds of objects in the Universe, all of which appear to be receding at a rapid clip. Considering that we’re not at a special place in the Universe, this observation is best explained by every object drifting away from every other object. Given that gravity is also present in our Universe, some other force must be acting upon each and every object in our Universe in order to counteract gravity and keep them on their paths to an increasingly isolated Universe. 

This force which counteracts gravity has been dubbed Dark Energy, and exactly what it is and how it operates is still extremely poorly understood. However, based on our observations, it must make up about 68% of all the energy present in the Universe to be able to do what we observe it doing - pushing all galaxies which aren’t tied to each other by gravity further apart from each other.

A representation of the evolution of the universe over 13.77 billion years. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. Image credit:  NASA / WMAP Science Team

A representation of the evolution of the universe over 13.77 billion years. More recently, the expansion has begun to speed up again as the repulsive effects of dark energy have come to dominate the expansion of the universe. Image credit: NASA / WMAP Science Team

It’s one thing to have an expanding Universe. If the expansion occurred at a fixed rate, we would draw our Universe expanding as a straight line in diagrams like the one above. However, we have found that the expansion of our universe is happening at an increasingly rapid pace. Our Universe’s expansion is accelerating, not constant. This means that if I watch two galaxies separating now, from some kind of Universal bird’s eye view, and then came back in a billion years, and watched another set of two galaxies expanding away from each other, the second set would separate much faster than the first set. 

Over a long enough period of time, this increasing speed of expansion means that the density of objects within the Universe will decrease. If every galaxy is increasingly distant from every other galaxy, images of galaxies outside our own Milky Way will also become increasingly out of date, as the light travel time also increases. If we pursue the increasing isolation of galaxies to its logical extreme, we arrive at an end-of-Universe scenario called “heat death”. Heat death arrives when a galaxy runs out of gas to form new stars, and the stars which remain are overwhelmingly either very faint red, brown, and black dwarf stars, black holes, or neutron stars. With no new gas able to arrive into the galaxy, the galaxy must end its star formation. Once the remaining red dwarfs and other stellar objects radiate away the last of their heat, and the entire Universe has reached a single, even temperature, we have arrived at the death of heat in our Universe.  This is currently our Universe’s forecast for its eventual end state - and a direct consequence of having such a large amount of Dark Energy, pressing our Universe outwards into an ever-faster expansion.

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Is Dark Energy Pushing Our Galaxy Somewhere?

If dark energy is pushing our Galaxy in a known and straight line, then where is it pushing us to? What kind of force is driving the dark energy and where is it taking us to? Or is there a force that is drawing dark energy to it, and we are just trapped within it?
Still from an animation illustrating the accelerating expansion of the universe due to dark energy. Image credit: NASA's Goddard Space Flight Center Conceptual Image Lab

Still from an animation illustrating the accelerating expansion of the universe due to dark energy. Image credit: NASA's Goddard Space Flight Center Conceptual Image Lab

Originally posted on Forbes!

Dark energy isn't pushing our Galaxy in a specific direction; it's responsible for the expansion of the space between all objects in space which are not tied to each other by gravity.

So, for instance, dark energy doesn't influence the distance between the Sun and the Earth, because our gravitational ties are much stronger than the gentle stretching that space is doing. Similarly, our Sun isn't being moved relative to the center of our Galaxy, because the force of gravity binding us to the Galaxy is much stronger than what dark energy can exert.

Dark energy can't even shear apart the gravitational ties which attach our Milky Way to the Andromeda Galaxy and the numerous, tiny dwarf galaxies which hover around our Galaxy in their own orbits. The distances here are enormous; Andromeda is 2.5 million light years away. Light arriving to us from Andromeda now will have left that galaxy when our planet Earth had only just seen its first humans.

Dark energy is a force to reckon with only for galaxies much more distant from us, where the gravitational force between our Milky Way Galaxy and that faraway galaxy plays no role. It's often phrased as a gravitational counter-force, but that's only partially correct. It is true that dark energy seems to have a repellent influence on the space surrounding it, but unlike gravity, which is strongest around concentrations of mass in the Universe, dark energy seems to be evenly spread throughout the universe, with not a care for the presence of a galaxy, planet, or supercluster . It's this evenhandedness of dark energy that means that gravity can overpower it in densely populated regions of the Universe.

In this artist's conception, dark energy is represented by the purple grid above, and gravity by the green grid below. Gravity emanates from all matter in the universe, but its effects are localized and drop off quickly over large distances. Image credit: NASA/JPL-Caltech

In this artist's conception, dark energy is represented by the purple grid above, and gravity by the green grid below. Gravity emanates from all matter in the universe, but its effects are localized and drop off quickly over large distances. Image credit: NASA/JPL-Caltech

Dark energy is not a directional force - there's no bulk motion to the left, right, or an arbitrarily defined up that this expansion leans towards. So there's no point to which the universe is being drawn, and equally there's no origin point from which the expansion is unspooling. Any given point in space is simply, and very gradually, becoming more distant from most other points in the universe. It's not that our Galaxy is being pushed around- dark energy is instead ballooning out the space within which our Galaxy sits.

Where does dark energy come from? So far that's a mystery; we can measure its influence to a great degree of confidence, but if we knew the exact nature of why it behaves the way we observe it to, we'd probably rename it something less vague than 'dark energy'.

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