Even At Its End, The Universe Will Never Reach Absolute Zero
Universe

Even At Its End, The Universe Will Never Reach Absolute Zero

If you dare, imagine the final end of the Universe. The stars have all burned out, past, present, and future. Stellar corpses such as neutron stars and white dwarfs have radiated the last of their remnant energy, fading to black and ceasing to emit any radiation at all. The great gravitational dance of masses within galaxies has ended, with each mass either inspiraling into a black hole or being ejected into the intergalactic medium. And the last remaining structures will decay as black holes evaporate due to Hawking radiation, and dark energy drives every unbound structure from every other such structure that it isn’t bound to.

At this point, the Universe will be cold and empty, with the density of matter and radiation effectively zero. However, our Universe contains dark energy: an energy that is inherent in the fabric of space itself. According to our best observations, dark energy does not appear to degrade, which means that even if the Universe continues to expand indefinitely, this form of energy density will remain constant. Surprisingly, no matter how long we wait, this fact alone will keep our Universe’s temperature from dropping to absolute zero. Here’s the science behind it.

Matter-and-radiation-filled Universes governed by General Relativity cannot remain static.

In a Universe governed by General Relativity, filled with matter-and-energy, a static solution is not possible. That Universe must either expand or contract, with measurements revealing very quickly and decisively that expansion was correct. Since its discovery in the late 1920s, there have been no serious challenges to this paradigm of the expanding Universe. NASA / GSFC

Our story begins in the early days of modern cosmology, with the publication of Einstein’s General Relativity. A Universe governed by Einstein’s principles could not, according to popular belief, be filled with nearly equal amounts of material everywhere while remaining stable and the same size. For generations, it was commonly assumed that the Universe was static and eternal, offering an unchanging “stage” for matter in the Universe to engage in its cosmic performance. However, as Einstein’s new theory of gravitation gained popularity, many realized that this assumption was physically impossible.

If General Relativity governs your Universe and it is filled with a roughly equal density of “stuff” everywhere — where “stuff” can include any and every form of energy that is possible, including normal matter, black holes, dark matter, radiation, neutrinos, cosmic strings, field energy, dark energy, and so on — your Universe has only two options: expand or contract. Every other solution is unstable and will begin expanding or contracting after an infinitesimal amount of time, depending on your initial conditions.

The original 1929 observations of the Hubble expansion of the Universe, plus modern data.

The original 1929 observations of the Hubble expansion of the Universe, followed by subsequently more detailed, but also uncertain, observations. Hubble’s graph clearly shows the redshift-distance relation with superior data to his predecessors and competitors; the modern equivalents go much farther. All the data points towards an expanding Universe. ROBERT P. KIRSHNER (R), EDWIN HUBBLE (L)

We began measuring individual stars in other galaxies in the 1920s, proving their presence outside of the Milky Way and their massive, multi-million (or possibly multi-billion) light-year distances from Earth. We could measure the redshift of that light by measuring the spectrum of the light coming from those galaxies — breaking the light up into individual wavelengths and identifying absorption and emission lines from atoms, molecules, and ions — and by what multiplicative factor each individually identifiable line was shifted by.

When we collected that data in the late 1920s, a feat accomplished independently first by Georges Lemaître, then by Howard Robertson, and eventually (and most famously) by Edwin Hubble, it pointed to an unambiguous conclusion: the Universe was expanding. This was then put together into the modern Big Bang framework, with the discovery of the cosmic microwave background (a remnant bath of radiation from the hot, dense early phases of the Universe) putting the last nail in the coffin of possible competing alternatives.

The Holmdel Horn antenna and the original discovery of the cosmic microwave background.

According to the original observations of Penzias and Wilson, the galactic plane emitted some astrophysical sources of radiation (center), but above and below, all that remained was a near-perfect, uniform background of radiation, consistent with the Big Bang and in defiance of the alternatives. NASA / WMAP SCIENCE TEAM

The science of physical cosmology had two key measuring goals from the 1960s through the 1990s.

  • To measure the Hubble constant, H0, which tells us how fast the Universe is expanding now.
  • To calculate what we called the deceleration parameter, q0, which told us how quickly a distant galaxy seemed to recede from us as time went on.

The concept is straightforward: the equations that govern the Universe specify a relationship between the matter-and-energy currently within it and how the rate of expansion will change over time. If we can measure the expansion rate now and how quickly it evolves, we can not only determine what makes up the Universe, but also its previous history and future fate. As additional telescopes and observatories were built, and huge breakthroughs in instrumentation happened, our answers became more accurate as well as more exact.

Distances versus redshifts in the expanding Universe, along with best-fit cosmologies.

When we plot out all the different objects we’ve measured at large distances versus their redshifts, we find that the Universe cannot be made of matter-and-radiation only, but must include a form of dark energy: consistent with a cosmological constant, or an energy inherent to the fabric of space itself. NED WRIGHT’S COSMOLOGY TUTORIAL

There is a critical relationship between our Universe’s expansion rate and its fate in a Universe loaded with matter and radiation. Consider the Big Bang to be the starting gun for the ultimate cosmic race: between gravity, which works to recollapse the Universe and bring everything back together, and the initial rate of expansion, which works to force everything apart. You can imagine a variety of outcomes:

  • One where gravity wins, and overcomes the expansion, causing the Universe to recollapse and end in a Big Crunch,
  • One where the expansion wins, where gravity is insufficient, and the Universe expands forever, with its density eventually dropping to zero,
  • One where gravity wins, and overcomes the expansion, causing the Universe to recollapse and end in a Big Crunch, 

However, when the crucial data came, it pointed to none of these. Instead, gravitation fought against the initial expansion, causing distant galaxies to recede from us at a slower and slower rate, and then something unusual happened. Around 6 billion years ago, these distant, receding galaxies began to move away from us at incredible speeds. The expansion of the Universe was accelerating in some way.

The different possible fates of the Universe, with our actual, accelerating fate included.

The different possible fates of the Universe, with our current, accelerating fate shown on the right. After enough time goes by, the acceleration will leave every bound galactic or supergalactic structure completely isolated in the Universe, as all the other structures accelerate irrevocably away. We can only look to the past to infer dark energy’s presence and properties, which require at least one constant, but its implications are larger for the future. NASA & ESA

Today, 13.8 billion years after the Big Bang, it is clear that the Universe contains not only many forms of matter and radiation, but also an unexpected component: dark energy. When we look at the modern Universe, we observe it in one of its most fascinating states: after an enormous number of interesting, luminous, large-and-small-scale structures have evolved, but before dark energy has driven them all away from us to practically imperceptible distances.

We observe stars forming, living, and dying in today’s Universe; galaxies and galaxy clusters colliding and merging; new planets being formed; but we also see these distant objects moving away from one another. After enough time has passed:

  • Stars will only develop through the rare and infrequent merger of failed or extinct stars,
  • All of the bright stars will consume their fuel,
  • The energy of star remains will be radiated away,
  • A considerable proportion of masses will be swallowed by black holes,
  • All of the remaining individual masses will be gravitationally ejected by galaxies,
  • The leftover radiation from the Big Bang will redshift to arbitrarily low energies,
  • Every single black hole will eventually evaporate,

All the while, the Universe continues to expand relentlessly due to dark energy.

Scenarios of a Universe dominated by matter, radiation, or dark energy.

A Universe that expands will exhibit different properties if dominated by matter, radiation, or dark energy. While matter and radiation both get less dense over time, causing a Universe dominated by those components to expand more slowly over time, a Universe dominated by dark energy (bottom) will not see the expansion rate drop, causing distant galaxies to appear to accelerate away from us. E. SIEGEL / BEYOND THE GALAXY

At the level of individual particles, certain incredibly long-term effects may occur that are well beyond our ability to measure. Protons can decay, but modern experiments have limited the proton’s lifetime to less than 1025 times the age of the Universe. Atomic nuclei may undergo quantum tunneling to achieve a more stable configuration, such as iron-56 or nickel-60. And improbable but not forbidden events, like the ionization of matter due to a stray, energetic photon, may eventually kick all of the electrons off of atoms and ions.

However, at some time in the future, any arbitrarily large part of the Universe will be completely empty: devoid of all forms of normal matter, dark matter, neutrinos, or any of the radiation that currently permeates the Universe. Even the Big Bang’s huge thermal bath of photons will change to long wavelengths, low densities, and energy that asymptotes to zero. All that will be left is the energy inherent in space itself – dark energy — and the consequences it brings.

The possible fates of the Universe: a big rip, big crunch, or constant dark energy.

The far distant fates of the Universe offer a number of possibilities, but if dark energy is truly a constant, as the data indicates, it will continue to follow the red curve, leading to the long-term scenario described here: the eventual heat death of the Universe. However, the temperature will never drop to absolute zero. NASA / GSFC

Surprisingly, one of the consequences of a Universe with a cosmological constant — the type of dark energy best supported by data, in which the energy density of dark energy remains constant across time and throughout all of space — is that the Universe’s temperature does not reach zero. Instead, the Universe will be filled with a bath of very low-energy radiation that will appear everywhere but at a temperature of 10-30 K. (Compare that to the cosmic microwave background today, which is more like ~3 K, or some 1030 times hotter.)

We can begin to understand why by considering black holes. Because observers close to the event horizon and observers further away from the event horizon differ on what the ground state of the quantum vacuum is, black holes evaporate. The more severely space is curved towards a black hole’s event horizon, the greater the difference an observer there will experience for the quantum vacuum.

An illustration of heavily curved spacetime, outside the event horizon of a black hole.

An illustration of heavily curved spacetime, outside the event horizon of a black hole. As you get closer and closer to the mass’s location, space becomes more severely curved, eventually leading to a location from within which even light cannot escape: the event horizon. PIXABAY USER JOHNSONMARTIN

However, quantum fields are continuous throughout all of space, and there are potential light routes that connect any location outside the event horizon to any other location outside the event horizon. The difference in zero-point energy of space between those two locations tells us, as initially derived in Hawking’s landmark 1974 study, that radiation will be emitted from the region surrounding the black hole, with the event horizon playing an important role. The temperature of that radiation will be determined by the mass of the black hole (lower-mass black holes would have higher temperatures), and it will have a complete blackbody spectrum.

In a Universe with a cosmological constant, we don’t have an event horizon, but we do have a different kind of horizon: a cosmological horizon. Two observers in distant places will be able to communicate at the speed of light for a limited time. They will eventually recede from one another so quickly that a light signal emitted by one will never reach the other, similar to how a signal emitted by us today could only reach an observer 18 billion light-years away. They can only receive “older” signals from us, and we can only get “older” light from them.

The size of our visible Universe (yellow), along with the amount we can reach (magenta).

The size of our visible Universe (yellow), along with the amount we can reach (magenta). The limit of the visible Universe is 46.1 billion light-years away, as that’s the limit of how far away an object that emitted light that would just be reaching us today would be after expanding away from us for 13.8 billion years. However, beyond about 18 billion light-years, we will never be able to access a galaxy even if we traveled towards it at the speed of light. E. SIEGEL, BASED ON WORK BY WIKIMEDIA COMMONS USERS AZCOLVIN 429 AND FRÉDÉRIC MICHEL

Einstein’s equivalence principle is the key that unlocks the entire puzzle: the idea that observers cannot tell the difference between gravitational accelerations and any other sort of acceleration of equal magnitude. If you’re in an enclosed rocket ship and you feel yourself pulled down towards one end, you cannot know whether you’re pulled down because the rocket is at rest on Earth or because the rocket is accelerating in the “up” direction.

Similarly, it makes no difference whether you have an event horizon or a cosmic horizon; it makes no difference if a point mass (like a black hole) or dark energy (like a cosmological constant) is accelerating two observers relative to one another. The physics is the same in either case: a consistent amount of heat radiation is emitted. Based on the current value of the cosmological constant, a blackbody spectrum of radiation with a temperature of 10-30 K will always permeate all of space, no matter how far into the future we go.

A black hole produces Hawking radiation outside the event horizon.

Just as a black hole consistently produces low-energy, thermal radiation in the form of Hawking radiation outside the event horizon, an accelerating Universe with dark energy (in the form of a cosmological constant) will consistently produce radiation in a completely analogous form: Unruh radiation due to a cosmological horizon. ANDREW HAMILTON, JILA, UNIVERSITY OF COLORADO

Even at its end, no matter how far into the future we travel, the Universe will constantly emit radiation, ensuring that it will never reach absolute zero. However, this final-state photon bath should be extremely difficult to observe. This cosmic radiation, with a temperature of 10-30 K, should have a wavelength of 1028 meters, or nearly 30 times the size of the observable Universe today.

It may be a long journey to the end, but if our current understanding of the Universe is true, even empty space, no matter how far into the future we wish to travel, can never be completely empty.

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