The universe could stop expanding 'remarkably soon', study suggests

After nearly 13.8 billion years of nonstop expansion, the universe could soon grind to a standstill, then slowly start to contract, new research published in the journal Proceedings of the National Academy of Sciences suggests.

In the new paper, three scientists attempt to model the nature of dark energy — a mysterious force that seems to be causing the universe to expand ever faster — based on past observations of cosmic expansion. In the team's model, dark energy is not a constant force of nature, but an entity called quintessence, which can decay over time.

The researchers found that, even though the expansion of the universe has been accelerating for billions of years, the repellent force of dark energy may be weakening. According to their model, the acceleration of the universe could rapidly end within the next 65 million years — then, within 100 million years, the universe could stop expanding altogether, and instead it could enter an era of slow contraction that ends billions of years from now with the death — or perhaps the rebirth — of time and space.

And this could all happen "remarkably" quickly, said study co-author Paul Steinhardt, Director of the Princeton Center for Theoretical Science at Princeton University in New Jersey.

"Going back in time 65 million years, that's when the Chicxulub asteroid hit the Earth and eliminated the dinosaurs," Steinhardt told Live Science. "On a cosmic scale, 65 million years is remarkably short."

Nothing about this theory is controversial or implausible, Gary Hinshaw, a professor of physics and astronomy at the University of British Columbia who was not involved in the study, told Live Science. However, because the model hinges on past observations of expansion alone — and because the present nature of dark energy in the universe is such a mystery — the predictions in this paper are currently impossible to test. For now, they can only remain theories.

Energy of the void

Since the 1990s, scientists have understood that the expansion of the universe is speeding up; the space between galaxies is widening faster now than it was billions of years ago. Scientists named the mysterious source of this acceleration dark energy — an invisible entity that seems to work contrary to gravity, pushing the universe's most massive objects farther apart rather than drawing them together.

Though dark energy makes up approximately 70% of the total mass-energy of the universe, its properties remain a total mystery. A popular theory, introduced by Albert Einstein, is that dark energy is a cosmological constant — an unchanging form of energy that's woven into the fabric of space-time. If that's the case, and the force exerted by dark energy can never change, then the universe should continue expanding (and accelerating) forever.

However, a competing theory suggests that dark energy doesn't need to be constant in order to fit with observations of past cosmic expansion. Rather, dark energy may be something called quintessence — a dynamic field that changes over time. (Steinhardt was one of three scientists who introduced the idea in a 1998 paper in the journal Physical Review Letters.)

Unlike the cosmological constant, quintessence can be either repulsive or attractive, depending on the ratio of its kinetic and potential energy at a given time. Over the last 14 billions years, quintessence was repulsive.  For most of that period, though, it contributed insignificantly compared to radiation and matter to the expansion of the universe. That changed about five billion years when quintessence became the dominant component and its gravitational repulsion effect caused the expansion of the universe to speed up.

"The question we're raising in this paper is, 'Does this acceleration have to last forever?'" Steinhardt said. "And if not, what are the alternatives, and how soon could things change?"

The death of dark energy

In their study, Steinhardt and his colleagues, Anna Ijjas of New York University and Cosmin Andrei of Princeton, predicted how the properties of quintessence could change over the next several billion years. To do this, the team created a physical model of quintessence, showing its repellent and attractive power over time, to fit with past observations of the universe's expansion. Once the team's model could reliably reproduce the universe's expansion history, they extended their predictions into the future.

"To their surprise, dark energy in their model can decay with time," Hinshaw said. "Its strength can weaken. And if it does so in a certain way, then eventually the antigravitational property of dark energy goes away and it transitions back into something that's more like ordinary matter."

According to the team's model, the repellent force of dark energy could be in the midst of a rapid decline that potentially began billions of years ago.

In this scenario, the accelerated expansion of the universe is already slowing down today. Soon, perhaps within about 65 million years, that acceleration could stop altogether — then, within as few as 100 million years from now, dark energy could become attractive, causing the entire universe to start contracting. In other words, after nearly 14 billion years of growth, space could start to shrink.

"This would be a very special kind of contraction that we call slow contraction," Steinhardt said. "Instead of expanding, space contracts very, very slowly."

Initially, the contraction of the universe would be so slow that any hypothetical humans still alive on Earth wouldn't even notice a change, Steinhardt said. According to the team's model, it would take a few billion years of slow contraction for the universe to reach about half the size it is today.

The end of the universe?

From there, one of two things could happen, Steinhardt said. Either the universe contracts until it collapses in on itself in a big "crunch," ending space-time as we know it — or, the universe contracts just enough to return to a state similar to its original conditions, and another Big Bang — or a big "bounce" — occurs, creating a new universe from the ashes of the old one.

In that second scenario (which Steinhardt and another colleague described in a 2019 paper in the journal Physics Letters B), the universe follows a cyclical pattern of expansion and contraction, crunches and bounces, that constantly collapse and remake it. If that's true, then our current universe may not be the first or only universe, but just the latest in an infinite series of universes that have expanded and contracted before ours, Steinhardt said. And it all hinges on the changeable nature of dark energy.

How plausible is all this? Hinshaw said the new paper's interpretation of quintessence is a "perfectly reasonable supposition for what the dark energy is." Because all of our observations of cosmic expansion come from objects that are millions to billions of light-years away from Earth, current data can only inform scientists about the universe's past, not its present or future, he added. So, the universe could very well be barreling toward a crunch, and we'd have no way of knowing until long after the contraction phase began.

"I think it really just boils down to how compelling do you find this theory to be and, more importantly, how testable do you find it to be?" Hinshaw added.

Unfortunately, there is no good way to test whether quintessence is real, or whether cosmic expansion has started to slow, Steinhardt admitted. For now, it's just a matter of fitting the theory with past observations — and the authors do that capably in their new paper. Whether a future of endless growth or rapid decay awaits our universe, only time will tell.

Originally published on Live Science.


  1. In the beginning it was void and the energy of God movd in the void...behold the universe were folded like a scroll. ..then.. I saw a new universe and old has passed away....
    The universe will melt in fervent heat (not cold death)

  2. In my opinion,
    If we want to understand the expansion of the universe and dark energy, it is necessary to change the model of the universe a little bit.
    There really is no such thing as dark energy.
    What exists is a completely natural phenomenon.

    Current universe models describe the Big Bang as the release of all baryonic energy from a dimensionless singular point. The expansion is also based on this.

    However, if we consider Space (the part which has no Time) as a sea composed of neutral=non-vibrating energy units (quantum), the model to be obtained produces the same results.
    The Big Bang would be defined as the breaking of symmetry as a result of a forceful impact (by an unknown reason), in a point in this sea.
    As the shock wave of this impact propagates spherically-symmetrically, it will mobilize the stagnant energy units in the Space (Still there is no The Time). Each unit will occupy a larger area with its vibration. This will be the initial impetus for expansion.
    This global expansion will be transformed with increasing violence (as more units will join this new system).

    Secondly, the energy density of the area where the symmetry is broken will increase. This shows us two different densities of media.
    Thus, a denser medium will begin to expand into a field of ​​less density.
    (If they want to test this as an experiment, let them hold a water tanker full of pure water under light pressure for a while. When the water molecules reach the lowest entropy, let them pump one beat of high heat-energy from a central point.

    I think, If we watch the movement of the heated water molecules, similar results can observe. (If the temperature is too high, the first spherical expansion will turn into a torus expansion to maintain the surface-to-volume ratio.)
    If the medium change of a shock wave is examined, it is seen that a reflection wave goes backwards during the transition. The frequency and wavelength of this wave are equivalent to the expanding wave. Only the amplitude and phase are different.
    (I think the same thing will happen in a heat shock wave propagating in water. A water molecule vibration field width will be wavelength of the wavefront of propagating impact as the same.)

    These reflected echo waves will have a concave slope as they travel. That is, as they progress, their energy per unit field will increase.
    When they reach the center, they will create a driving source that will support the expansion.
    These echo waves will also preserve the integrity of the particles that condensed and solidify (tiny ice) during the first symmetry breaking.

    In our universe, I think these waves are the equivalent of Time. Space and Time became should be intertwined in this way.

    I have some explanations about various subjects (such as; Special relativity, dark matter, antimatter, gravity, etc.) compatible with this assumption in my answers at the quora site.

  3. Another thing to worry about! I'm going to line my house with aluminum foil to protect myself from any bad effects from this.

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