The Sciences

Why Won't the Gravity of Galaxies Keep Our Universe From Expanding?

About five billion years ago, the universe switched gears and began to accelerate instead of slowing down.

This is an edition of Quintessence, a series about fundamental ideas in science.

To look deeper into space is to look longer back in time. By peering at far-flung galaxies, we can reconstruct the moments of the universe’s birth.

In 1929, Edwin Hubble measured how fast other galaxies in the universe were moving away from our own, changing cosmology forever. His results ushered in an era when scientists studied the universe as a dynamic, expanding entity, not as a big, unchanging thing. The idea of an expanding universe also lent credence to the Big Bang theory, according to which an event created the space, time and stuff of our universe 13.8 billion years ago.

Scientists figured that shortly after this event – the ‘bang’ of the Big Bang – the universe began to expand outward until the gravity of the galaxies coming to life inside it would have slowed it down. In 1998, two independent groups of scientists observed the light from very distant supernovae and found something strange. The light from the supernovae was dimmer than expected, which means they were farther out than a decelerating universe would suggest. Other scientists did the math and found that about five billion years ago, the universe switched gears and began to accelerate instead of slowing down.

This conclusion flew in the face of conventional wisdom and marked an important turn in our understanding of the universe. The evidence required scientists to explain what could be acting against the gravitational pull of galaxies. Scientists called this mysterious entity dark energy.

Also read: Why Scientists Are Confused About How Fast the Universe Is Expanding

Let’s visualise our expanding universe. Think of a flat sheet of rubber with little dots on its surface, each one representing a galaxy. As we stretch the sheet, the dots on the rubber move further away from each other. The universe expands in a similar manner. Just forget about the edges of the sheet and replace the expansion of a 2D surface by a uniform expansion in three dimensions.

The space between galaxies expands and pushes galaxies apart. As a result, the concentration of matter becomes more dilute. Radiation gets stretched out to longer wavelengths. Without dark energy, these changes could have decelerated and eventually stopped – but the universe’s expansion is actually accelerating (at a constant rate).


The most popular interpretation of dark energy is that it is the energy associated with empty space, i.e. vacuum energy. In this definition, dark energy is evenly distributed in space, and doesn’t clump together like ordinary matter does (due to gravity). And because it’s a property of space itself, dark energy doesn’t become less potent as space expands, allowing it to accelerate the universe’s expansion.

The idea of dark energy as vacuum energy also neatly fits into the general theory of relativity, which is what a theoretical cosmology works with to understand how the universe works. This theory includes a set of equations that describe how matter curves the fabric of space and time, and how that curvature affects the dynamics of matter in return. Vacuum energy appears in the equation for space-time curvature as a fixed parameter that Albert Einstein, the theory’s progenitor, called the cosmological constant.

However, dark energy isn’t necessarily constant in its strength or scale. Physicists have developed alternate models of dark energy in which, for example, it as a field of energy permeating the universe and whose properties change over time. In this scenario, dark energy is like a smooth and fine mesh in space, each point on which has some energy. This mesh, technically called quintessence, slowly evolves as the universe ages.

Because dark energy doesn’t clump together, acts everywhere and doesn’t interact directly with ordinary matter, scientists have thus far not been able to spot it. It has been two decades since the vacuum energy interpretation of dark energy took hold, and we’re nowhere close to figuring out what it is or how it came to be. What we do know is that irrespective of the role dark energy played in shaping our universe when the latter was young, it will play a decisive part in determining how our universe will end.

If dark energy is a constant and remains that way, the universe will keep expanding until it experiences a heat death. Physicists also call this likely scenario the Big Chill or the Big Freeze.

As galaxies are pushed further apart from each other, they will also begin to wither because they won’t have enough matter in their surroundings to replenish themselves. Stars will run out of fuel, explode and eventually disperse as gas clouds. Black holes will evaporate – slowly, but surely. After a sufficiently long period of time, the universe will be devoid of structure or order, and its temperature will be barely above absolute zero. After even more time, the universe will become a cold, desolate nothingness and stay that way forever.

Also read: As the Universe Expands, Earth Grows Lonelier

But if we assume that dark energy’s potency is not constant, more explosive doomsday scenarios become possible. For example, some physicists argue that in the quintessence scheme, dark energy could become more potent as the universe expands further. If this happens, space will expand forcefully enough to rip entire galaxies apart and, eventually, destroy atoms and decompose subatomic particles. The end result: all matter shredded to bits, the very fabric of spacetime torn apart. This is the Big Rip.

However, if dark energy becomes less potent as the universe expands, it could switch sides, team up with gravity and end the universe in a reverse Big Bang event, called the Big Crunch. Space will get squished together, compressing matter and shrinking the wavelength of all energetic radiation everywhere, effectively slow-cooking the cosmos.

There are a few ways to end the universe without dark energy. Some evidence suggests our universe – in the form of the subatomic particles it contains, their properties and the properties of the forces acting between them – is currently in a metastable state. If the universe’s energy could be raised, it could be pushed into a more stable state with different properties that may not be conducive to the existence of life. Think of it like rolling the ball in the image below from the higher valley it’s in to the lower, more stable, valley.

In one even more complicated scenario, a small part of the universe could tunnel through the energy barrier and into the lower valley by some quantum mechanical trickery. The rest of the universe, suddenly exposed to these more stable conditions through the tunnel, could quickly succumb to a rapidly expanding bubble of change and completely collapse within microseconds.

One way or another, the universe is bound for horrific tragedy – or at least it would be if we had any way of knowing we’re right about these things. Physicists created each of these scenarios based on cosmological data that’s hardly incontestable. In fact, and as if to illustrate the magnitude of uncertainties, cosmologists are currently locked in debates about how fast the universe is really expanding and what its shape really is.

We’re far from resolving the most fundamental features of our universe; the end is a long, long way away – in our imagination as well as in time. A heat death will conservatively take about 10100 years, and a vacuum death bubble, at least 1058 years. A Big Rip or a Big Crunch – whichever is set to happen – is 10 billion years in the future, if not more. Our Sun will be long gone by then, and the Solar System with it.

It’s hard to say if any humans will be able to witness any of these events. But that doesn’t mean we can’t ever know: we can by unravelling the mysteries of dark energy. To do this, we need to collect light from very distant galaxies, pointing our telescopes billions of years back in time.

Ronak Gupta is doing a PhD in fluid mechanics at the University of British Columbia, Vancouver.