## Q: Does anti-matter really move backward through time?

Physicist: The very short answer is: yes, but not in time-traveler-kind-of-way.

There is a “symmetry” in physics implied by our most fundamental understanding of physical law, and is never violated by any known process, that’s called the “CPT symmetry“.  It says that if you take the universe and everything in it and flip the electrical charge (C), invert everything as though through a mirror (P), and reverse the direction of time (T), then the base laws of physics all continue to work the same.

Together, the PT amount to  putting a negative on the spacetime position, $(t,x,y,z)\to(-t,-x,-y,-z)$.  In addition to time this reflects all three spacial directions, and since each of these reflections reverses parity (flips left and right), these three reflections amount to just one P.  You find, when you do this (PT) in quantum field theory, that if you then flip the charge of the particles involved (C), then overall nothing really changes.  In literally every known interaction and phenomena (on the particle level), flipping all of the coordinates (PT) and the charge (C) leaves the base laws of physics unchanged.  It’s worth considering these flips one at a time.

Charge Conjugation Flip all the charges in the universe.  Most important for us, protons become negatively charged and electrons become positively charged.  Charge conjugation keeps all of the laws of electromagnetism unchanged.  Basically, after reversing all of the charges, likes are still likes (and repel) and opposites are still opposites (and attract).

Time Reversal If you watch a movie in reverse a lot of nearly impossible things happen.  Meals are uneaten, robots are unexploded, words are unsaid, and hearts are unbroken.  The big difference between the before and after in each situation is entropy, which almost always increases with time.  This is a “statistical law” which means that it only describes what “tends” to happen.  On scales-big-enough-to-be-seen entropy “doesn’t tend” to decrease in the sense that fire “doesn’t tend” to change ash into paper; it is a law as absolute as any other.  But on a very small scale entropy becomes more suggestion than law.  Interactions between individual particles play forward just as well as they play backwards, including particle creation and annihilation.

Left: An electron and a positron annihilate producing two photons. Right: Two photons interact creating an electron and a positron.  We see both of these events in nature routinely and they are literally time-reverses of each other.

Parity If you watch the world through a mirror, you’ll never notice anything amiss.  If you build a car, for example, and then build another that is the exact mirror opposite, then both cars will function just as well as the other.  It wasn’t until 1956 that we finally had an example of something that behaves differently from its mirror twin.  By putting ultra-cold radioactive cobalt-60 in a strong magnetic field the nuclei, and the decaying neutrons, were more or less aligned and we found that the electrons shot out (β radiation) in one direction preferentially.

Chien-Shiung Wu in 1956 demonstrating how difficult it is to build something that behaves differently than its mirror image.

The way matter interacts through the weak force has handedness in the sense that you can genuinely tell the difference between left and right.  During β (“beta minus”) decay a neutron turns into a proton while ejecting an electron, an anti-electron neutrino, and a photon or two (usually) out of the nucleus.  Neutrons have spin, so defining a “north” and “south” in analogy to the way Earth rotates, it turns out that the electron emitted during β decay is always shot out of the neutron’s “south pole”.  But mirror images spin in the opposite direction (try it!) so their “north-south-ness” is flipped.  The mirror image of the way neutrons decay is impossible.  Just flat out never seen in nature.  Isn’t that weird?  There doesn’t have to be a “parity violation” in the universe, but there is.

Matter’s interaction with the weak force is “handed”.  When emitting beta radiation (a weak interaction) matter and anti-matter are mirrors of each other.

Parity and charge are how anti-matter is different from matter.  All anti-matter particles have the opposite charge of their matter counterparts and their parity is flipped in the sense that when anti-particles interact using the weak force, they do so like matter’s image in a mirror.  When an anti-neutron decays into an anti-proton, a positron, and an electron-neutrino, the positron pops out of its “north pole”.

CPT is why physicists will sometimes say crazy sounding things like “an anti-particle (CP) is like the normal particle traveling back in time (T)”.  In physics, whenever you’re trying to figure out how an anti-particle will behave in a situation you can always reverse time and consider how a normal particle traveling into the past would act.

“Anti-matter acts like matter traveling backward in time”. Technically true, but not in a way that’s useful or particularly enlightening for almost anyone to know.

This isn’t as useful an insight as it might seem.  Honestly, this is useful for understanding beta decay and neutrinos and the fundamental nature of reality or whatever, but as far as your own personal understanding of anti-matter and time, this is a remarkably useless fact.  The “backward in time thing” is a useful way of describing individual particle interactions, but as you look at larger and larger scales entropy starts to play a more important role, and the usual milestones of passing time (e.g., ticking clocks, fading ink, growing trees) show up for both matter and anti-matter in exactly the same way.  It would be a logical and sociological goldmine if anti-matter people living on an anti-matter world were all Benjamin Buttons, but at the end of the day if you had a friend made of anti-matter (never mind how), you’d age and experience time in exactly the same way.  You just wouldn’t want to hang out in the same place.

The most important, defining characteristic of time is entropy and entropy treats matter and anti-matter in exactly the same way; the future is the future is the future for everything.

## Q: How do we know that everyone has a common anecestor? How do we know that someone alive today will someday be a common ancestor to everyone?

The original question was: From biology and genetics we know that any group of living organisms had a mitochondrial most recent common ancestor (mitochondrial Eve): a female organism who lived in the past such that all organisms in this group are her descendants.

How can we [theoretically] prove this? (I.e. without assumption that there’s equal probability to mate for any two individuals).

Also: can we prove that one of 3 billion women currently living on Earth will be the only ancestor of all human population some day in the future, and all other currently living women (except her mother and daughters) will have no descendants at that day?

Physicist:  The fact that everyone on Earth has a common female ancestor if you go back far enough is a direct consequence of the theory of common descent.  It looks like everything that lives is part of the same very extended family tree with a last universal common ancestor at its base.  In order to have two familial lines that never combine in the past you’d need to have more than one starting point for life, and all the evidence to date implies that there’s just the one.  Luckily, you don’t have to go all the way back to slime molds to find common ancestors for all humans; the most recent were standard, off-the-shelf people.

It turns out that animal and plant cells aren’t particularly good at producing usable energy, so before we could get around to the business of existing we needed to get past that problem.  The solution: fill our cells with a couple thousand symbiotic bacteria.  Literally, they’re not human; mitochondria reproduce on their own and have their own genetic code.  There’s a hell of a lot of communication and exchange of material between them and our cells, and without them there wouldn’t be an us, but they are (arguably) separate organisms that we are absolutely dependent on and which are completely dependent on us.

Here comes the important bit: eggs cells have mitochondria but sperm cells don’t, so mitochondria are passed strictly from mother to child.  There’s no implicit reason for your mitochondria and your father’s to be related at all.  The nice thing about that is that it keeps the genetic lineage very simple: all of your mitochondria are essentially clones of those in the egg cell you started as and (for our female readers) any of your children’s mitochondria will essentially be clones of yours.  The genes of sexually reproducing beings are a lot trickier to keep track of over time; every generation half of our genes are dumped and the other half are shuffled with someone else’s (which makes your DNA is unique).  The one real advantage to talking about mtDNA (mitochondrial DNA) is that you only have a single chain of ancestors to worry about.  By the way: you can do exactly the same thing with the Y-chromosome and direct male lines.

Mitochondria are passed only from mother to child, so if you follow your direct female line back, you’re following your mitochondria’s ancestors as well.

If you have a group of creatures with two types of mitochondria, two “haplogroups“, living under a population ceiling, then eventually one or the other will be bred out.  The math behind this is essentially the Drunkard’s Walk.  The number of folk in a haplogroup can increase or decrease forever, unless it gets to zero; given nough time and no where else to go (a population limit), eventually the drunkard’s walk will take him off a cliff (zero population).  So, if you start with a small village and several haplogroups, then after a few generations you’ll probably have fewer.

Why did that particular woman become the Mitochondrial Eve instead of one of the others?  Luck and fecundity.

That isn’t saying much.  It boils down to the rather fatalistic statement that “in order to be the last thing standing, you just have to wait for everything else to die off”.  We can’t prove that a woman today will eventually be declared, very post-mortem, the Mitochondrial Eve to everyone (that is; all but one haplogroup will die off).  But statistically: that’ll definitely happen.  To within less than a 1% error, every inherited line of every kind has died off; practically every species, sub-species, gene, haplogroup, whatever, has gone extinct leaving only the amazing scraps that remain.  That’s evolution in a nutshell: you chip away all the life that isn’t an efficient, functioning organism (and then a hell of a lot more besides) and the inconceivably tiny fraction that remains is (some of the) efficient, functioning organisms.  So, chances are that every living haplogroup presently around will go extinct eventually.  When there’s one haplogroup left, then you can say that they all have a common Mitochondrial Eve and when there are zero haplogroups left, then all human issues become moot.

In order to definitely not get a modern Mitochondrial Eve, you’d need human populations that are absolutely independent (and viable) forever.  Maybe if we colonized Mars and then completely forgot it?

So, if any given haplogroup eventually dies out, then why is there more than one?  Well good news: over long time scales (millennia) mtDNA accrues tiny changes through random mutation, leading to a relatively few distinguishable lineages.  We live in a kind of meta-family tree, where each branch is entire groups of female lines.  Even though some branches stop, others will randomly sprout new branches (“new” meaning “with mtDNA that’s detectably different at all”).

By reading the mtDNA of people from all over the world, you can track how folk have expanded across the planet.

In fact, by carefully looking at the differences in our mtDNA and theirs, we can show that Neanderthals are not a parent species of ours, but cousins, and the common “Eve” that we share with them lived around half a million years ago.  We can do the same thing with regular genetics and damn near any living thing to see how and how closely we’re related.

Humanity’s (present) Mitochondrial Eve is not our unique common ancestor nor is she our most recent common ancestor.  Mitochondrial Eve is merely the most recent ancestor of all living people by means of a direct female line alone.  If you allow for the inclusion of both men and women, then our most recent common ancestor jumps from around 120,000 – 150,000 years ago, for direct female lines only, to as recent as 3,000 years ago, for any ol’ lines.  There’s no way to even reasonably guess who or where any of these common ancestors were.  Probably lived near big population centers?  Maybe?

Looking at mitochondria is a solid, simple way of understanding evolution and inheritance, but it doesn’t paint an accurate picture of how genes move around populations.  An important fact to keep in mind is that a huge fraction of the people alive today will eventually be a common ancestor to all of humanity.  Even if your family doesn’t increase or decrease the population (every couple has two kids), your family is still going to grow exponentially (2 kids, 4 grandchildren, 8 great-grandchildren, …).  It only takes about 30 generations to have a billion descendants (less if you really work at it), so if you have kids, and they have kids, and so on, then in less time than you’d expect (not forever anyway) your genes will be spread thinly throughout all of humanity.  Many of your particular genetics won’t make it, but many of them will.  For example, if your haplogroup dies out then your mtDNA won’t be around, but the genes that dictate, say, the shape of your earlobe might end up all over the place.

Too true.

Arguably, that’s the reason for sex.  Maybe not the first reason most folk would cite, but they weren’t at the meeting a billion years ago.  By mixing our genes every generation we can prevent genetic lines from disappearing forever, which is good: more diversity means more combinations for evolution to try out in a pinch.  Almost as good, useful mutations and combinations of genes can be distributed and used by (a random subset of) the entire species after a mere few thousand years!  Sexual reproduction literally makes us much better at evolving.  Huzzah for doing it!

Posted in -- By the Physicist, Biology, Evolution, Probability | 3 Comments

## Q: Are some colors of light impossible? Can any color of light be made?

Physicist: Just so we can talk about this using physics rather than poetry, for the sake of this article “color” really means “frequency”.  Light frequency is a bit more objective than color and includes things we can’t see (like ultraviolet).

When you put gas in a tube and pass electricity through it you get light.  Electro-dynamically speaking, this is basically just beating the hell out of the atoms and letting the atoms ring like bells (only emitting light instead of sound).  Individual atoms are like simply shaped bells; the “tones” they make (or absorb) are very specific.  The colors emitted by atoms, their “spectra”, are different for different elements.  This is tremendously useful because it allows us to look at the light coming from something and immediately know what that thing is made of.

When given the energy (like in a neon light) atoms will emit light with very specific frequencies.  These are the lines for mercury, lithium, cadmium, strontium, calcium, and sodium that happen to fall in the visual spectrum.  There are many more lines that we mere humans can’t see.

Some colors fall into the gaps between the spectral lines of all elements (technically, almost all of them do).  So you can be forgiven for thinking that there are some colors that just never show up in nature.  Fortunately, there are a lot of effects that shift all those lines, blur them, or even split them.

Top: When a light source moves toward or away from you its spectrum is shifted up or down. Middle: In a hot gas the atoms are moving randomly, so whether the lines are Doppler shifted up or down is also random. This broadens the spectral lines. Bottom: With a very strong magnet you can change some of the electron’s energy levels and as a result transitions that would normally create the same color become separated.

So you can create any color by starting with a few distinct colors and then moving your light source either toward or away from an observer to Doppler shift one of your colors to the target color.  That’s a little like using a piano to get some notes, and then driving it around to get all the notes in-between.

An efficient means to play C above high C.

You can also just use a non-atomic source of light, like something that’s glowing hot, and then select out the color you want with a monochromator (the rest is chucked out).  But, as with any process that involves throwing out almost everything, this is remarkably inefficient.

Monocromators generate light with a single color (one might say “mono-chromatic” light) by just throwing away all the light that isn’t the right color.

So, say you want to create a very specific color of light with as little “waste light” as possible.  Well, a good place to start is lasers.  For some slick quantum reasons, the photons in laser beams are all kinda “clones” of each other; inside of any kind of laser device, the presence of the right kind of photon encourages the creation of other identical photons.  Pretty soon your laser is bubbling over with coherent, identical photons and not a lot else.  These share, among other things, a common color.

Laser beams: not a lot of colors.

It turns out that only a very small fraction of atomic spectral lines are good candidates for lasers.  It is possible to create laser light at any frequency between microwaves and X-rays, but the technique is a long way from efficient.  You can use the Doppler effect to change the color of your laser, but in order to make any significant change you’ll need to get it going a significant fraction of the speed of light.

If you want to efficiently create any very specific color of light, you just need to strap a laser to a starship.  So… no need to be picky.

An efficient means to make green light.

## Proxima B!

Physicist: Good news!  There’s an Earth-sized planet in the habitable zone of Proxima Centauri, a star so named because it’s the closest star to Earth and it’s in the constellation Centarus.  This isn’t a question anyone asked, but I thought it was worth mentioning: the closest star that could possibly host life is the closest star.

The new planet is called “Proxima B” because the naming convention says that’s what you should call the fist planet discovered around a star called “Proxima” (if/when another planet is discovered, it would be called “Proxima C”, then D, etc.).  This is hopefully a place holder until someone comes up with a better name, like Krypton or Xena.  Strictly because it’s a little cumbersome to refer to both the star, Proxima, and the planet, Proxima B, for the purposes of this post I do hereby declare that the newly discovered planet will be called “Bacchus”, after the god Bacchus.  (Bacchus: when you’re too drunk for organized religion, welcome to the Bacchanalia.)

The bright stars are Alpha Centauri A and B, which are both about the same size as our Sun.  Proxima Centauri (sometimes called Alpha Centauri C) is the nearly invisible red star that’s circled below them.  A and B orbit each other every 80 years (so they switch places about every 40) and Proxima may orbit the pair every half-million years.  This picture is very zoomed in, so to the naked eye these appear to be a single star.

Just for a rough sense of scale, the Earth is 1 AU from the Sun (this is how the “Astronomical Unit” is defined), Proxima is a little over a quarter million AU away from us and less than a tenth that distance from Alpha Centauri (a pair of stars it may-be/probably-is orbiting).  Bacchus orbits Proxima every 11 days in an orbit that’s a twentieth the size of ours (0.05AU).

Bacchus was discovered using the radial velocity technique.  As Bacchus orbits, its host star, Proxima, wobbles and we can detect its tiny back-and-forth movement using the Doppler Effect (things moving toward us appear slightly bluer and things moving away appear slightly redder).  This wobble method reveals how long the orbit is (11 days) and gives a rough idea of the mass of the planet (around 1.3 Earths, give or take), but unfortunately it tells us very little more.  If we were lucky enough that Bacchus’ orbit brought it between its star and us, then we’d be able to (possibly) get a look at its atmosphere and determine its size.  Unfortunately, like the vast majority of exoplanets, Bacchus’ orbital plane is pointing off in some random direction, so it never eclipses Proxima from our perspective.

A bird and a planet (Venus) transiting in front of the Sun. We can use alignments like this to study the atmospheres of other planets by looking at how sunlight/starlight filters through them.  Presumably we could study birds the same way, but it seems unlikely that anyone’s bothered to make the attempt.

So, beyond how much it weighs and where it is, we have no direct data on Bacchus.  But take heart!  We also have access to well-reasoned speculation!  Gas giants tend to be gigantic, so with only 1.3 (give or take) Earth masses to work with, Bacchus is almost certainly a rocky planet.  You’d expect that since Bacchus orbits so close to Proxima it would be hot, but Proxima is very dim (~1/10 the mass and ~1/10 the light of our Sun), so Bacchus should have more or less the temperature necessary for liquid water (the way Earth does).

Tight orbits also tend to lead to tidal locking.  Like our Moon’s relationship with Earth, Bacchus may have one side always pointed at Proxima.  It may literally have a dark side and a light side and if that’s the case, then there’s probably only a thin habitable region near the ring of twilight between the two.

We do know a lot about Proxima from telescopes.  In particular, it’s a flare star which means that its magnetic field plays a central role in its behavior; energy stored in its twisted up magnetic field is occasionally released as bursts of x-rays and solar flares.  That may mean that Bacchus’ atmosphere is already long gone.  This is something stars do to their planets (see Mercury and Mars, for example), it’s just that flares and x-rays are especially good at it and Bacchus is especially close.  That said, an already thick atmosphere and a healthy magnetic field go a long way toward preserving what air a planet has.  Fingers crossed.

There is some good news: Proxima is extremely stable.  Both our Sun and Proxima are about five billion years old, ours brightens over time and is due to burn out in another five billion years, but Proxima will still be doing what it does for the next few trillion (with a “t“) years.  When Proxima runs out of fuel the universe will be several hundred times older than it is now, so even if there’s no life on Bacchus (or any other planets around Proxima), there’s still an incomprehensible amount of time to get it right.  Things have a way of changing over a few trillion years (one would assume).

The bird/Venus/Sun picture is from here.

Posted in -- By the Physicist, Astronomy, Physics | 13 Comments

## Q: Is there anything unique about our solar system?

Physicist: Maybe.

As far as stars go, ours is a dull as dishwater, middle of the road, dime a dozen, main sequence star.  You can’t spit in space without hitting a Sun-like star.  So this question is really about the stuff around our Sun.  But, beyond their stars, comparing solar systems is a little tricky.

A solar system is basically a star (sometimes a couple of them) with a tiny bit of grit left over.  Not only does our Sun comprise 99.86% of the mass of our solar system, but it’s disproportionately “loud”.  While the Earth may have a smattering of radio antennae pumping signals into space, the Sun is a screaming ball of electromagnetic noise almost a million miles across.  And not for nothing: it’s seriously bright.  So finding stuff around other stars is difficult, not just because everything other than stars is tiny, but because of the stars themselves.  In fact, most of the techniques we have for detecting stuff around other stars involves looking at the effect of said stuff on their host stars.  Ultimately, if you really want to get a good look at other solar systems you’ve got to get off of your podunk planet and go there.

That said, we are now living in the century of planetary discovery.  Every civilization has known about 6 planets: Mercury, Venus, Earth, Mars, Jupiter, and Saturn.  Uranus, Neptune, and Ceres were discovered in the 19th century.  Pluto and the first few exoplanets were discovered in the 20th century.  But in the first decade and a half of this century we’ve discovered dozens of new dwarf planets in our solar system and thousands of exoplanets in other solar systems.  The rate of discovery is ramping up exponentially.

The planets that are easiest to detect are massive (vertical axis) and close to their stars (horizontal axis).  We would probably miss most of our planets (blue) if we were looking for them from another solar system.

The methods we use to detect planets around other stars are much better at detecting planets that are big (which isn’t surprising) and close to their stars.  So the easiest planets to find are “hot Jupiters“.  But based on what we’ve seen so far it looks like damn near every star in the sky, even binary and trinary stars, have planets in orbit around them.  Despite the difficulties in finding any planets, we’ve confirmed hundreds of solar systems with multiple planets (which strongly implies that practically all of them have multiple planets).  And that’s based on methods that look at only a tiny fraction of the sky and miss the majority of planets.

In this picture our solar system is in the center and the exoplanets we’ve found are red dots.  The majority of the exoplanets have been found using the Kepler space observatory (guess where it’s pointing).  The point is: we’ve discovered thousands of planets around other stars and we’ve barely scratched the surface.

Even better!  By far the most common element in the universe is hydrogen with oxygen a distant third.  So we can expect to find H2O (water) all over the universe, including on exoplanets, and when we look that’s exactly what we find.  That doesn’t necessarily mean that there’s life out there, but if it’s around it’ll definitely have lots of places to do its living.

So here’s the point.  It’s really hard to see planets around other stars, we miss most of them, and we’ve only looked at a tiny fraction of the nearby stars.  And yet!  We’re finding planets freaking everywhere.  Big, small, hot, cold, rocky, gaseous, short years, long years, dry, or wet.  The shocking variety and preponderance of planets has completely rewritten our ideas about planetary and solar system formation.  We once thought that solar systems would all more or less “follow the same script” during their formation and end up as variations on our own.  Instead we find that each is surprising and unique.  Our solar system is unique like a snowflake in a blizzard is unique.

Some podunk planet.

The top two pictures are from the (free as of writing) exoplanet app and that picture of Earth is from space.

Posted in -- By the Physicist, Astronomy, Philosophical | 5 Comments

## Q: What is dark energy?

Physicist: The universe is expanding and the rate of that expansion is accelerating.  If the universe were full of only matter (both regular and dark) and energy, then we’d expect that the expansion would be slowing.  Dark energy is the thing that is responsible for that acceleration.  Now as for what exactly that thing is: we have interesting and vaguely informed guesses.

Back in the day Einstein came to a few realizations after he spent some time thinking about falling elevators and rockets.  This lead to a way of describing the nature of gravity (falling and whatnot) in terms of the geometry of spacetime and a relation between that geometry and the amount of matter and energy that’s around.  Einstein’s field equations can be used to describe how time and space are influenced by the presence of big chunks of matter like planets and stars, manages to explain/predict Mercury’s weird orbit and a bucket of other stuff, and has passed every test it’s been put to with flying colors.  That last bit is important: it’s easy to come up with crazy new theories, but hard to come up with theories that precisely predict results that were not previously understood.

Enter Friedmann, who happened to be thinking about the entire universe one day.  Einstein’s field equations related mass/energy with the shape of spacetime.  So, pondered Friedmann, what happens when you apply those equations to the universe as a whole?  First he assumed that space can expand or contract over time (why not?), then he threw that assumption at Einstein’s field equations to see if they’d stick.  Somewhat surprisingly, this is easier than it sounds.

Mr. Алекса́ндр Алекса́ндрович Фри́дман (Alexander Friedmann), a man who looks exactly like you’d hope he would.

On a totally over-the-top-large scale (billions of lightyears) the matter and energy of the universe is distributed roughly uniformly.  On this scale we describe the matter and energy in the universe as the “cosmological fluid”, because on the largest scales even the largest clumps of matter are as indistinguishable as atoms in water.  These “largest chunks” (as far as we know) are galactic super clusters; ours is about half a billion lightyears across and there are possibly many millions of super clusters in the observable universe.  And that’s only the observable universe.  Guessing the size of the entire universe based only on the part of it that we can see is like trying to guess the size of the Earth by standing in a featureless open field.

So you’re somewhere.  How big is it?

Words utterly fail when trying to describe how much “out there” is out there.  Point is: all that’s important for figuring out the behavior of the universe as a whole is the average density of matter and energy, which is roughly the same everywhere.  You don’t need to stress about every star and galaxy in the universe, or even about the size of the universe as a whole, any more than you need to stress about every grain of sand in a pile of sand.

When Friedmann said space can expand or contract over time he said it this way: $ds^2 = \left[a(t)\right]^2dx^2-c^2dt^2$.  That’s the spacetime interval (which is how distance is measured in spacetime) with the addition of a scaling term, a(t).  If a(t) doubles, that means that the distance between any two points in space has doubled.  So, if you figure out how a(t) changes over time, you can describe how the universe is expanding or contracting over time.  Incidentally, a(t) also describes the cosmological redshift of light; if a(t) doubles between when some light is emitted and absorbed, then the wavelength of that light (like the space it’s traveling through) will double and longer wavelengths mean redder.  The oldest light in the universe was emitted when a(t) was about 1100 times smaller than it is now.

It turns out (this is not obvious) that Einstein’s equations dictate that $a(t) = kt^\frac{2}{3(w+1)}$, where w is the weirdly named “equation of state” and k is a constant number.  $w=\frac{1}{3}$ and $a(t)\propto t^\frac{1}{2}$ for a universe filled with radiation (light, neutrinos, and matter moving near light speed).  $w=0$ and $a(t)\propto t^\frac{1}{3}$ for a universe filled with regular matter (including you, your stuff, basically everything else, and dark matter too).  In both of these cases the universe expands, and the rate of that expansion slows down, forever.

As space expands the density of matter and radiation decreases, because there’s more space for it to be spread out in.  But radiation not only gets more spread out, but redshifted as well and that means that the energy density of radiation drops faster than the energy density of ordinary matter.  So, we can expect that w, which describes a combination of both matter and radiation, should be somewhere in the range $0\le w\le \frac{1}{3}$.  Since the expansion of space decreases the energy density of radiation more than matter, over time w should drift closer to 0 as matter becomes more dominant.

The expansion of space decreases the energy density of matter by spreading it out and decreases the energy density of radiation by spreading it out as well as redshifting it. But the expansion of space doesn’t decrease the energy density of dark energy; instead it just seems to create more

But here’s the thing!  Since the late 90’s we’ve been able to show that the expansion of the universe is speeding up, not slowing down.  In order for that to happen the equation of state of the universe must be $w\le-\frac{1}{3}$.  This came as a bit of a shock to cosmologists.  But being brow-beaten by experiment and observation is what good science is all about.  Onward.

Upon closer examination of the expanding universe we find that including “stuff” with the equation of state w=-1 is a good fit.  This corresponds to a uniform negative energy with a constant density.  There are a couple of things about that which are… a little strange.  A constant density means that as space expands there’s more of this stuff around.  Matter and radiation are indeed being thinned out more and more by the expansion of the universe, but this very bizarre new stuff doesn’t get thinned out.  The fact that it has negative energy is just icing on the weirdness cake.

Not knowing what else to call it, physicists have dubbed this uniform, undiluting, negative-energy stuff “dark energy”.  That’s not to say that we have any idea what it is, but that’s no reason to not give something a name.  Unfortunately, dark energy and dark matter are difficult to study (hence the names) so it’s tricky to figure out exactly how much of each is around, but of the energy and matter around today roughly 75% is dark energy, 20% is dark matter, and 5% or less is regular matter.  Over time the percentage of dark energy should approach 100% as the expansion of space waters down all of the matter and radiation.

We can infer the existence of dark matter in large part because models of the universe that take it into account do a good job of describing the observed evolution of the universe, while models that don’t take dark energy into account do a terrible job.  Because the amount of dark energy seems to be proportional to the amount of space around, it seems fairly reasonable to say that dark energy is the “energy of empty space”.  What in the hell that’s supposed to mean is now a lively topic of discussion and debate.  There are a bunch of theorists and experimentalists running around trying to directly detect and/or describe dark energy, and with any luck they just might do it.

Posted in -- By the Physicist, Astronomy, Physics, Relativity | 28 Comments