Infinity makes physicists uncomfortable. This makes sense if you think about one of the most fundamental “laws” of physics: the second law of thermodynamics, which states that in a closed system, the total entropy of a system will always increase.
Thermodynamics is what is called a stochastic theory, and as such gets into all kinds of details about state configurations and potential distributions, but at its core, the second law can be thought of as a way of saying that over time, other forms of energy get converted into heat (which is a form of light) and dissipates.
There are always local violations of this law — a star survives because hydrogen fusion produces more energy than it consumes, and life is arguably an example where order exists contrary to the general disorder of the universe, but each of these are snapshots. Stars, for instance, go into a red giant phase because enough helium (which needs more energy to fuse) has gathered to make helium fusion possible, which releases considerably more energy.
However, this process does not hold true forever. Eventually, a star produces iron nuclei, which actually take more energy to create than they release. The resulting implosion (as hydrostatic pressures no longer oppose the power of gravity) creates an implosion and then a shockwaved induced explosion that blows about half the star into an expanding shell of hydrogen gas and particles and leaves behind a star of degenerate matter in the process.
Now, for (most) astronomers and physicists, degenerate has a very specific meaning. Degenerate here means that the internal core has created a state where most of remaining atoms (typically lithium and above) have been compressed so that they form an intricate lattice which traps the bath of electrons associated with those atoms. This matter is very dense, but it is also no longer fusing, so the only heat comes from either from the emission of legacy photons or of legacy electrons.
As these electrons slip out of the lattice, the white dwarf begins to crystalize from the inside out. A newborn white dwarf is hot, but it’s basically radiating already generated heat. Over time, with no fuel to replace that heat, the star begins to cool, becoming red, then brown, then finally “black”. The white dwarf is tiny — typically about the size of the Earth, or about a million times smaller than when it was in its main stream sequence, yet still retains about 60% of the mass of the sun. This makes such white dwarfs very dense, sufficiently dense in fact, that up to a certain size, the more massive a white dwarf is, the smaller it is.
Now, the Sun is about halfway through its life-cycle, and, in about 5 billion years (or about 18 billion years after the Big Bang) it will become a white dwarf through this exact process, alternately growing and shrinking as it exhausts specific fuels. As the Sun ages, it also will naturally get hotter, to the extent that within 1.5 billon years, the Earth’s oceans will probably have evaporated (see The Oceans of Earth), so humans only have about another 500 million years to get out to the stars before the planet becomes uninhabitable. Note, this is still a long time — 500 million years ago, the most advanced life forms on the planet were trilobites, giant pill bugs— but astronomically 500 million years is about 3% of the age of the universe.
Note that once the Sun does become a white dwarf, it will only take about a billion years to go from slightly hotter than the sun is now to about a quarter of that, but the cooling process will go on for a long time. In 10 trillions years (roughly 1000 times the current age of the universe or CAU), the remnant white dwarf will be a crystal about five degrees above absolute zero. Ironically enough, this also is about the time that it will take most Red Dwarves (which are simply smaller normal stars) to become white dwarves. In other words, in a thousand million years, most of the stars left in the universe will be red dwarfs and white dwarfs.
Now, before going too far into the future, it’s worth looking at the outliers in the degenerate space. The Sun is close to the upper limit masswise for becoming a white dwarf (it is actually not quite an average star, but is instead perhaps about one and a half standard deviations on the big side). If the Sun had about 40% more mass, it would become a blue carbon dwarf — quite literally, a diamond in the sky. Of course such a blue carbon dwarf would weigh in at something like 10,000,000,000,000,000,000,000,000, 000,000,000 carats, and would be so dense that something the size of a wedding band setting would weigh as much as the Himalaya mountain range.
About 1.44 solar masses, a star becomes a neutron star. This is actually doubly degenerate, because such a star effectively turns matter into a Bose-Einstein condensate — multiple nuclei can occupy the same space at the same time. As a consequence, neutron stars are very small but very, very heavy. Like a ballet dancer doing a pirouette, this translates into an angular momentum on the order of thousands of revolutions a second. Moreover, because most stars have clear electromagnetic fields, these neutron stars generate immense magnetic fields that show up as brilliant jets, often several hundreds of light years long.
One step up from neutron stars are quark stars. In effect, except for a thin crust of “ordinary” degenerate matter, the boundaries between neutrons have broken down to the extent that the star is a sea of free-floating quarks. Such stars are hypothetical — no known quark star has ever been observed. In addition to this, an additional kind of quark star might be made up partially or fully from strange quarks, which are potentially stable and which converts other quarks to strange quarks. Strangelets have actually been proposed as a way of creating planet destroying bombs, as even a small amount of strange material would eventually turn an ordinary planet into amorphous strange goo within a disturbingly short amount of time (months or even weeks).
The final beast in the exotic star zoo is of course the Black Hole. Black holes are from type IA supernovas, but they can also form from accretion (which may in fact be the primary way of making them). Black holes have been postulated since Einstein, (and such scientists as Chandrasekhar, John Wheeler and the late Stephen Hawkings have expended considerable research on both their origins and their fate).
Black holes do not have infinite gravitational attraction. They have a very high density, and very likely look, within their event horizon, like very dense quark stars. What is so fascinating about a black hole, though, is that event horizon. Once you pass this event horizon, no amount of energy will let you cross back over the event horizon in the other direction. It‘ s a cosmic trap, made possible because gravitational curvature at that point is such that light itself cannot escape.
There’s a lot of confusion about what exactly that event horizon is. It is not the actual radius of the star. Instead, the event horizon radius (called the Schwarzchild Radius) is proportional to the mass of the stellar object, which raises the possibility that a neutron star close to the minimal side of a black hole (more than 2.3 times the mass of the sun) may actually have a Schwarzchild radius within the boundary of the star itself.
Once you start getting into galactic black holes, which may have a million solar masses or more, the radius will be roughly one million times the diameter of the Sun — the radius being directly propostional to the mass. Intriguingly, that still makes the event horizon only about a light year or so across, and the actual density within (at least parts of) the event horizon might well be roughly comparable to the density of space near Earth.
Typically a black hole will only really be visible when it is eating another star, and then primarily by the accretion disk (and the magnetic jets) as powerful magnetic fields move charged particles to the poles. There have been some theories postulated that at the event horizon, there should be sufficient energy to create a spherical “wall of fire” from the dissociation of matter, but in the absence of evidence, this remains only a hypothesis.
Additionally, there have been theories proposed that a black hole, if both charged and spinning, that could effectively expose the singularity, the point of theoretically infinite density that marks the center of the black hole. There have finally been arguments made (with more than some justification) that there is a domain of quantum gravitational mathematics that has not yet been developed that holds sway most obviously at the upper extremes of relativity, just as Einstein’s General Theory of Relativity didn’t prove Isaac Newton false, only that it was only accurate within what were then measurable domains.
Most science fiction fans would wish that Einstein too was wrong (or at least was just not sufficient at the extremes of our knowledge), because it would imply that faster than light travel was possible. Yet ultimately this comes down to the emerging conflict between how the quantum world works and how relativity works. Despite a century of looking, we still do not have a quantum theory of gravity, though the recent repeated observation of gravity waves provides a tantalizing glimpse of potentially new tools to help shed light on what that theory might look like.
What emerges is an idea that there are no infinities, no true singularities, out there, only equations that imply they are because they do not have all of the information they need. This also informs an understanding of the end of space and time.
In the Five Ages of the Universe authors Fred Adams and Gregory Laughlin introduced an interesting way of describing time — the cosmological decade. Using a scale from -50 to 50, each decade represented 10 to the nth power years, so that the -50th decade represented the first 1/1⁰⁴³ seconds (which not coincidentally is also the smallest unit of Planck Time, based upon Heisenberg’s uncertainty principle. It’s the shortest meaningful unit of time, and as such can be thought of as the “tick” of the universe’s clock.
By that scale, we are currently early in the 10th cosmological decade (or 13.772 billion years) in what is the Baryonic era. We have a mixture of baryonic mass (protons, neutrons, electrons and so forth) and weakly interacting dark matter (from neutrinos through to the Higgs particle and beyond). Star formation is still underway, and likely will be for the next two ‘decades’, though these are shifting from massive primordial stars made up mostly of hydrogen through to stars like the Sun which contain the remnants of elements formed in prior supernovae to the end of the era, where the dominant form of regular star will be red dwarfs, and even stars the size of our Sun will be rarities.
The end of this era will see the start of the degenerate era (by the late 13th to 14th decade). Star formation will have ended, most planets will have either been consumed by their host stars or flung into the depths of space. The local group of galaxies will have merged together, with vast amounts of space between to the next such group, and the expansion of the universe (still accelerating) will create a very different view of the universe. The sky will appear sparser, and the deep space portrait showing galaxies deep in the sky will instead be mostly empty outside of the super galaxy that’s all part of the local cluster. Rather than brilliant pinpoints of light, these will be subtle dots of crimson darkness against a profound black, as the last glow of the big bang fades even to the most sensitive instruments.
By the 30th cosmological decade, some 10 billion billion billion years from now. Most baryonic matter (and quite probably most dark matter) is within massive stellar-group sized black holes. What isn’t will disappear 10 “decades” later as protons decay via quantum tunneling. What’s more, the apparent universe at that point will appear to shrink as universal expansion causes what’s left to disappear beyond an event horizon.
Now, this is actually where things get very interesting. First note that we have event horizons on both sides — we are outside of the event horizons of black holes, but the expanding universe creates an event horizon of its own beyond which nothing outside can affect what happens inside. This symmetry suggests that there may in fact be something very profound about event horizons and their relationship of the universe. What’s more, when the density of space becomes too low, there are no longer clocks, because there is no longer anything to “tick”.
Eventually, the universe accelerates to the point where the scale of the event horizon is roughly within a few orders of magnitude of the Planck length. This means that the probability that a particle will emerge rises dramatically, and the amount of time that the particle can then create anti-particles also rises, while the likelihood of at least one of those particles moving beyond the Planck length within the Planck Time approaches certainty. Once a particle exists beyond the duration of its Planck time, it may very well start a cascade of other particles that start doing the same thing, reversing the pressures that had been acting for cosmological decades and creating what amounts to a new Big Bang, along with a massive expansion in the amount of matter in the universe that cannot find antimatter fast enough.
Could this be happening now? Possibly, though I suspect the conditions really only can occur when a given space is quite literally empty, and the event horizon of the universe is too large for that to occur in what is comparatively speaking early days. In this particular way of looking at things, the universe is not really expanding. Rather, the event horizon of the universe is shrinking, something which may be the same thing, but it also may not be. If, as I suspect, the nature of gravity is in fact tied into the universal event horizon, then there is a limit beyond which gravity has no effect (likely tied into Planck’s constants).
The big uncertainty in this really comes down to a whole realm of physics that we have hints of, but no definitive understanding of — the world of dark matter. WIMPs, or Weakly Interacting Massive Particles, have been suggested by the Standard Particle Model, with the discovery a couple of years ago of a particle that is likely the Higgs Boson. The Higgs is especially interesting because it seems to be a key component of inertial mass — the mass which is affected by gravity. Whether it is the mediator of gravity waves (as the photon is considered the mediator of light waves) is still unknown, though is strongly suspected.
As such the role of WIMPS in the overall universe is very uncertain, as is their quantum mechanical properties. Presumably they have antimatter version. More than likely they do interact with black holes, and may be responsible for the odd, web-like nature of galactic clusters. They seem to influence the formation of galaxies, as the outer portions of most galaxies seem to spin faster than their visible mass can account for. Curiously, one of the oldest galaxies ever found did not have much dark matter (yet) and so while it clearly had star formation, it was remarkably tenuous. This raises the possibility that galaxies attract dark matter over time, rather than forming around it.
One final theory about the end of time, though the evidence to support it is thin, is the notion that the universe formed when two branes — multidimensional manifolds — interacted with one another. Branes are predicted as solutions to one set of equations from string theory, which assumes that multiple universes may exist that sporadically vibrate back and forth until they come into contact, the energy from one brane getting transferred to the other through the mechanisms we would perceive as “Big Bangs”.
This may also be another way of saying that there is a possibility that the energy level within a portion of the universe would drop below the Planck energy level, at which point, the “vacuum” collapses to a lower energy state. is The energy released from this is sufficiently potent to create a black hole bubble with a new universe on the inside, one that would replace the existing universe. This phase shift would occur at the speed of light, which means that there would be no way of perceiving this false vacuum collapse because the light from that event would arrive with the event itself. This may be another way of stating that the event horizon of the universe itself is collapsing (or, more precisely, that the quantum likelihood of a phase shift rises as the universe’s event horizon gets smaller).
Both of these scenarios are (potentially) ways of describing the Big Breakup. If one of the properties of space time is that there is an opposite fifth force to gravity that’s actually repulsive, it could be that this force (currently so small as to be unnoticeable) will eventually reach a scale where it exceeds the force not only of gravity, but of the strong-electromagnetic-weak force. Plasma matter (the stuff of which we are made of) gets pulled apart, and eventually, even dark matter disintegrates.
At one point, cosmologists considered the idea that the universe might actually slow its acceleration, and all of matter would begin to get drawn back together again by gravity (a scenario known as the Big Crunch). Given recent observations, this seems increasingly unlikely, save for the fact that black holes will either all merge or move beyond the event horizon of the perceived universe (and hence will be insufficiently affected by gravity to merge, even in the very longest time scales). At that time scale, ironically, time becomes meaningless, because nothing of significance will happen. There will be no clocks of any sort.
Fortunately for us, the universe today is still remarkably vibrant and fascinating. I for one intend to enjoy our moment in the sun.
Kurt Cagle has been fascinated by cosmology from the time he was a child, and only regrets that he won’t be able to live for billions of years because he wants to know what happens. He lives in Issaquah, WA, and writes computer courses and novels in his spare time.