How the Sun *really* shines

You never would’ve guessed that nuclear physics could be this easy.

“Mr. Burns: Smithers, hand me that ice-cream scoop.
Smithers: Ice-cream scoop?
Mr. Burns: Damn it, Smithers! This isn’t rocket science, it’s brain surgery!”

-The Simpsons

The Sun is the one object that’s out-of-this-world that everyone on Earth is familiar with. With a mass that’s some 300,000 times our entire planet’s worth, it’s the most powerful source of heat, light, and radiation in the Solar System by far.

Image credit: composite of 25 images of the Sun, showing solar outburst/activity over a 365 day period; NASA / Solar Dynamics Observatory / Atmospheric Imaging Assembly / S. Wiessinger; post-processing by E. Siegel.

The amount of energy it emits is literally astronomical. Here are some fun facts about the Sun:

  • It emits 4 × 10^26 Watts of power, or as much energy as ten quadrillion high-powered power plants would emit running full-bore at once.
  • It’s been shining for 4.5 billion years, emitting energy at a nearly constant rate the entire time. (Changing under 20% over that entire time frame.)
  • The energy emitted comes from Einstein’s famous E=mc^2, as matter gets turned into energy in the Sun’s core.
  • And finally, that core energy needs to propagate to the Sun’s surface, a journey that requires it to pass through 700,000 kilometers of plasma.

That last step is a lot of fun! Because photons collide with ionized, charged particles very easily, it takes somewhere around 170,000 years for a photon created in the Sun’s core to make it to the surface.

Image credit: Center for Science Education, via

Only then can it leave the Sun and light up the Solar System, our planets, and the Universe beyond. We’ve talked about why the Sun shines (and how we know it works) before, but we never talked about how that all-important step — how its mass gets converted into energy — in detail before.

At a macro level, it’s pretty simple, at least as far as nuclear physics goes.

Image credit: Michael Richmond of R.I.T., via

The way nuclear fusion works in the Sun — and in all except the absolute most massive stars — is by fusing humble protons (hydrogen nuclei) into helium-4 (nuclei with two protons and two neutrons), releasing energy in the process.

This might puzzle you slightly, as you may remember that neutrons are ever so slightly heavier than protons.

Image credit: Bernadette Harkness of Delta College, via

Nuclear fusion only releases energy when the mass of the products — of the helium-4 nucleus, in this case — is less than the mass of the reactants. Well, even though helium-4 is made up of two protons and two neutrons, these nuclei are bound together, which means that their combined mass of the whole is lighter than the individual parts.

Image credit: Nuclear Energy & Technology at Greenwood College, via

In fact, not only is helium-4 lighter than two protons and two neutrons individually, it’s lighter than four individual protons! It isn’t by all that much — just 0.7% — but with enough reactions, it adds up quickly. In our Sun, for example, somewhere around a whopping 4 × 10^38 protons fuse into helium-4 every second in our Sun; that’s how many it takes to account for the Sun’s energy output.

But it’s not like you can just turn four protons into helium-4; in point of fact, you never get more than two particles colliding at the same time. So how, then, do you build up to helium-4? It might not proceed how you expect!

Most of the time, when two protons collide together, they simply do just that: collide, and bounce off one another. But under just the right conditions, with high enough temperatures and densities, they can fuse together to form a state of helium you’ve probably never heard of: a diproton, made up of two protons and no neutrons.

The overwhelming majority of the time, the diproton — an incredibly unstable configuration — simply decays back into two protons.

But every rare once-in-a-while, less than 0.01% of the time, this diproton will undergo beta-plus decay, where it emits a positron (the electron’s antiparticle), a neutrino, and where the proton transmutes into a neutron.

To someone who was only viewing the initial reactants and the final products, the diproton lifetime is so small that they’d only see something like the diagram below.

Image credit: Nick Strobel of Astronomy Notes, via

So you wind up with deuterium — a heavy isotope of hydrogen — a positron, which will immediately annihilate with an electron, producing gamma-ray energy, and a neutrino, which will escape at a speed indistinguishable from the speed of light.

And making deuterium is hard! In fact, it’s so difficult that even at a temperature of 15,000,000 K — which is what we achieve in our Sun’s core — those protons have a mean kinetic energy of 1.3 keV apiece. The distribution of these energies is Poisson, meaning that there is a small probability of having protons with extremely high energies, and speeds rivaling the speed of light. With 10^57 protons (of which maybe a few times 10^55 are in the core), I get the highest kinetic energy a proton is likely to have is about 170 MeV. This is almost (but not quite) enough energy to overcome the Coulomb barrier between protons.

But we don’t need to overcome the Coulomb barrier completely, because the Universe has another way out of this mess: quantum mechanics!

Image credit:, via

So these protons can quantum tunnel into a diproton state, a small (but important) fraction of which will decay into deuterium, and once you make deuterium, it’s smooth sailing to the next step. While deuterium is only a slightly energetically favorable state compared to two protons, it’s far easier to take the next step: to helium-3!

Image credit: Plasma Physics at University of Helsinki, via

Combining two protons to make deuterium releases a total energy of about 2 MeV, or about 0.1% of the mass of the initial protons. But if you add a proton to deuterium, you can make helium-3 — a much more stable nucleus, with two protons and one neutron — and that’s a reaction that releases 5.5 MeV of energy, and one that proceeds far more quickly and spontaneously.

While it takes billions of years for two protons in the core to fuse together into deuterium, it takes only about a second for deuterium — once it’s created — to fuse with a proton and become helium-3!

Image credit: Antonine Education, via

Sure, it’s possible to have two deuterium nuclei fuse together, but that’s so rare (and protons are so common in the core) that it’s safe to say 100% of the deuterium that forms fuses with a proton to become helium-3.

This is interesting because we normally think of fusion in the Sun as “hydrogen fusing into helium,” but in reality, this step in the reaction is the only lasting one that involves multiple hydrogen atoms going in and a helium atom coming out! After that — after helium-3 is made — there are four possible ways to get to helium-4, which is the most energetically favorable state at the energies achieved in the Sun’s core.

Image credit: Caryl Gronwall of Penn State, via

The first and most common way is to have two helium-3 nuclei fuse together, producing a helium-4 nucleus and spitting out two protons. Of all the helium-4 nuclei made in the Sun, some 86% of them are made by this path. This is the reaction that dominates at temperature below 14 million Kelvin, by the way, and the Sun is a hotter, more massive star than 95% of stars in the Universe.

Image credit: Morgan-Keenan-Kellman spectral classification, by wikipedia user Kieff; annotations by me.

In other words, this is by far the most common path to helium-4 in stars in the Universe: two protons quantum mechanically make a diproton that occasionally decays into deuterium, deuterium fuses with a proton to make helium-3, and then after about a million years, two helium-3 nuclei fuse together to make helium-4, spitting two protons back out in the process.

But at higher energies and temperatures — including in the innermost 1% of the Sun’s core — another reaction dominates.

Image credit: Wikimedia commons user Uwe W., edited by me.

Instead of two helium-3 nuclei merging together, helium-3 can merge with a pre-existing helium-4, producing beryllium-7. Now, eventually, that beryllium-7 will find a proton; because it’s unstable, however, it might decay into lithium-7 first. In our Sun, typically the decay to lithium happens first, and then adding a proton creates beryllium-8, which immediately decays to two helium-4 nuclei: this is responsible for about 14% of the Sun’s helium-4.

But in even more massive stars, proton fusion with beryllium-7 happens before that decay to lithium, creating boron-8, which decays first to beryllium-8 and then into two helium-4 nuclei. This isn’t important in Sun-like stars — accounting for just 0.1% of our helium-4 — but in the massive O-and-B-class stars, this may be the most important fusion reaction for producing helium-4 of all.

And — as a footnote — helium-3 can in theory fuse directly with a proton, producing helium-4 and a positron (and a neutrino) straightaway. Although it’s so rare in our Sun that less than one-in-a-million helium-4 nuclei are produced this way, it may yet dominate** in the most massive O-stars!

Image credit: Randy Russell, of the proton-proton chain fusion process.

So, to recap, the vast majority of nuclear reactions in the Sun, listing only the heaviest final product in each reaction are:

  • two protons fusing together to produce deuterium (about 40%),
  • deuterium and a proton fusing, producing helium-3 (about 40%),
  • two helium-3 nuclei fusing to produce helium-4 (about 17%),
  • helium-3 and helium-4 fusing to produce beryllium-7, which then fuses with a proton to produce two helium-4 nuclei (about 3%).

So it might surprise you to learn that hydrogen-fusing-into-helium makes up less than half of all nuclear reactions in our Sun, and that at no point do free neutrons come into the mix!

Image credit: Ron Miller of Fine Art America, via

There are strange, unearthly phenomena along the way: the diproton that usually just decays back to the original protons that made it, positrons spontaneously emitted from unstable nuclei, and in a small (but important) percentage of these reactions, a rare mass-8 nucleus, something you’ll never find naturally occurring here on Earth!

But that’s the nuclear physics of where the Sun gets its energy from, and what reactions make it happen along the way!

** — And that’s just considering the proton-proton chain; in more massive stars, the CNO-cycle comes into play, a way of making helium-4 with the aid of pre-existing carbon, nitrogen and oxygen, something that happens in all but the very first generation of massive stars!

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Next Story — Starts With A Bang Podcast #12: Exoplanets, beyond our Solar System and Proxima b
Currently Reading - Starts With A Bang Podcast #12: Exoplanets, beyond our Solar System and Proxima b

Starts With A Bang Podcast #12: Exoplanets, beyond our Solar System and Proxima b

For thousands upon thousands of years, we didn’t know whether the other stars in the Universe were even like our Sun, much less whether they had planets around them like we find in our Solar System. Over the past 25 years, however, that question has not only been answered, but we’ve discovered thousands of confirmed planets. Even more exciting, we’ve found that the star systems out there are similar to our own in some ways but tremendously different in others, and that there are already more than 20 rocky planets known that are at the right distance to have liquid water on their surface, given Earth-like atmospheres. This includes the closest star to our own: Proxima Centauri, whose planet ‘Proxima b’ just might be our first step into the Universe beyond our Solar System. Enjoy!

Next Story — Ask Ethan: How small is an elementary particle?
Currently Reading - Ask Ethan: How small is an elementary particle?

The size, wavelength and temperature/energy scales that correspond to various parts of the electromagnetic spectrum. Image credit: NASA and Wikimedia Commons user Inductiveload, under a c.c.a.-s.a.-3.0 license.

Ask Ethan: How small is an elementary particle?

What’s the difference between “point-like” and what we can actually state?

“When we think about the present, we veer wildly between the belief in chance and the evidence in favour of determinism. When we think about the past, however, it seems obvious that everything happened in the way that it was intended.” -Michel Houellebecq

If you take any amount of matter, no matter how small or how large, there are only two options for what it’s made up of: either it can be split into something smaller, or it’s truly fundamental and indivisible. For most of the 19th century, we thought that atoms were that fundamental, smallest entity, since the greek words itself, ἄτομος, literally means “uncuttable.” But we know better now, and can split the atoms into nuclei and electrons, and the nuclei can be further split not only into protons and neutrons, but those entities themselves can be decomposed into more fundamental quarks and gluons. But can they be split further, and how do we know their “size” at all? Patrick Moore wants to know, as he asks:

What do scientists really mean when they state an elementary particle’s size?

“Size” is a difficult concept, but quantum mechanics is here to help.

A pentacene molecule, as imaged by IBM with atomic force microscopy and single-atom resolution. Image credit: Allison Doerr, Nature Methods 6, 792 (2009).

What you’re looking at, above, is a picture — taken with a technique not so different from a traditional photograph — of the individual atoms within a relatively simple molecule. It’s the fact that light is a wave that enables objects of a certain size to be imaged, but not anything that’s too small. You see, because light has a characteristic wavelength, it can interact with anything that’s roughly the size of that wavelength or larger, but not smaller. This is:

  • why you need a relatively large antenna to pick up radio waves, since their long wavelengths require a substantial antenna to detect them,
  • why the “holes” in the door of your microwave keep the microwaves in, because the wavelength of microwaves is larger than the size of the holes,
  • and why tiny dust grains in space are good at blocking short-wavelength (blue) light, less good at blocking longer-wavelength (red) light, and why they’re totally transparent to even longer (infrared) radiation.
Visible (left) and infrared (right) views of the dust-rich Bok globule, Barnard 68. The infrared light is not blocked, as the dust grains are too small to interact with the long-wavelength light. Images credit: ESO.

If you want to measure the size of the smallest particles, you need photons with smaller and smaller wavelengths. Because of the relationship between a photon’s energy and wavelength — they’re inversely proportional — that means you need to go to higher and higher energies to probe the smallest scales of all.

The electromagnetic spectrum, and how a photon’s energy scales with wavelength. Image credit: Philip Ronan of English Wikipedia, under a c.c.a.-s.a.-3.0 license.

But photons aren’t the only way to go; it’s possible to use any particles at high energies to probe the size of matter. One of the funny rules of quantum mechanics in nature is that it isn’t just particles of light that act as waves, but any particles at all, including composite particles like protons and indivisible ones like (so far) the electron has proven to be. It’s by going to high energies and colliding with a stationary target that we can either determine the size of a non-fundamental “particle” by seeing when it splits apart, or determine that if a particle isn’t fundamental, it will only show that property “below” a certain size.

Electrons exhibit wave properties as well, and can be used to construct images or probe particle sizes just as well as light can. Image credit: Thierry Dugnolle, of an electron wave pattern after passing through a double slit.

This was the very technique that enabled us to determine that:

  • Atoms aren’t indivisible, but are made up of electrons and nuclei with a size of ~1 Å, or 10^–10 meters.
  • Nuclei themselves can be further split into protons and neutrons, each with a size of ~1 fm, or 10^–15 meters.
  • And if you bombard the particles inside protons and neutrons — the quarks and gluons — with high-energy particles, they don’t show any internal structure, just like electrons.

For each of the particles of the Standard Model, we’ve determined that if they have a composite nature, or a physical “size” that differs from point-like, it must be less than 10^–19 meters or so.

The sizes of composite and elementary particles, with possibly smaller ones lying inside what’s known. Image credit: Fermilab, via

We might not think of this as weird, but there was a time where people didn’t know quantum mechanics, but they did know Einstein’s famous equation: E = mc2. If you said that an electron has the charge you measure it to have, and the electric potential energy was responsible for its mass, you could derive a size for it, known as the classical electron radius. This turns out to be pretty small, and equal to:

But we know this is wrong! This even turns out to be substantially larger than the size of a proton, and is larger by more than a factor of 1,000 from our best constraints. In other words, the particles we find are truly quantum in nature, and that means — if we go to arbitrarily high energies — the truly fundamental ones should be point-like.

The particles and antiparticles of the Standard Model. Image credit: E. Siegel, from his book, Beyond The Galaxy.

So when we talk about the size of an elementary particle, we talk about the search for something truly fundamental. Are the standard model particle truly indivisible? If so, we should be able to keep on going to high and higher energies, and should discover nothing that differs from point-like behavior all the way up to the Planck energy, or down to distance scales of 10–35 meters. Below that distance scale, physics doesn’t give sensible predictions, but we keep approaching it. Perhaps along the way, we’ll find that some (or all) of these particles can be further broken down, or perhaps that they’re made up of strings or membranes, or, alternatively, that they’re simply points all the way down. But all we know to date, as far as the actual sizes of particles, are the sizes of the non-fundamental ones. Everything else is just an upper limit, and the search to get to smaller and smaller scales continues.

Submit your Ask Ethan questions to startswithabang at gmail dot com!

This post first appeared at Forbes, and is brought to you ad-free by our Patreon supporters. Comment on our forum, & buy our first book: Beyond The Galaxy!

Next Story — Was Earth born with life already on it?
Currently Reading - Was Earth born with life already on it?

A micron-scale view of very primitive organisms. Image credit: Eric Erbe, digital colorization by Christopher Pooley, both of USDA, ARS, EMU.

Was Earth born with life already on it?

Rather than life arising on Earth, did it already exist in space prior to our planet’s formation?

“When you arise in the morning, think of what a precious privilege it is to be alive — to breathe, to think, to enjoy, to love.” –Marcus Aurelius

If you asked a professional — a biologist, a fossil-hunter or a geologist — how old life on Earth was back in the 1970s, you would’ve gotten a very careful “I don’t know” answer. Going back before the rise of mammals, before birds, dinosaurs, reptiles, fish, crustaceans or even starfish and jellyfish — before the Cambrian explosion some 500–600 million years ago — we knew that Earth was inhabited. We knew we were a living planet, but the evidence was very scarce. While the past half-a-billion years or so provides a very rich fossil record, the way fossils form has an inherent limit to how far back we can see. Normally, animal corpses can get covered over by water, and by dirt deposits atop that water, creating the fossil record that we know, examine and study.

Trilobites fossilized in limestone, from the Field Museum in Chicago. Image credit: flickr user James St. John, under a cc-by-2.0 license.

That’s sedimentary rock: the kind that contains fossils. But place too many layers of rock atop your fossils for too long, and that combination of pressure and time will cause changes in that rock, and result in a metamorphosing of its contents. Rock that begins to metamorphose can still contain fossils so long as it’s only partially metamorphosed. But completely metamorphosed rock no longer has any. So if you asked a scientist who studied Earth’s natural history some 40 years ago how old life on Earth was, they would’ve told you at least one-to-two billion years and probably more, but they couldn’t prove it.

In general, exposing sedimentary rock to extreme pressures and/or high temperatures over long time periods will cause them to metamorphose, destroying any fossils inside. The metamorphic rock shown here, dating to about 1.9 billion years ago, is one such example. Image credit: flickr user James St. John, under a cc-by-2.0 license.

After all, it’s not like you can just go back in time and look at what was present then; the only evidence we have is the little bits and pieces that survive from back then, and almost all of what survives has changed over that time.

Hadean diamonds embedded in zircon/quartz, some of which date to over 4 billion years old. Image credit: Martina Menneken, Alexander A. Nemchin, Thorsten Geisler, Robert T. Pidgeon & Simon A. Wilde, via

But in the decades since, we realized something: even though the fossils themselves may no longer be discernible to us today, the remnants of organic matter leave a particular signature in the form of carbon. You may be used to “carbon dating” in the form of measuring the carbon-14 to carbon-12 ratio in organisms, since both forms of carbon are absorbed into organic matter, with carbon-14 being created in the upper atmosphere by cosmic rays and decaying with a half-life of around 5,700 years. As long as you’re alive, you breathe in and ingest both forms of carbon; when you decompose, the carbon-14 decays and isn’t replaced by any new carbon-14. Hence, if you can measure the carbon-14 to carbon-12 ratio (carbon dating), you can know roughly, with an error of a few thousand years, how long ago a particular organism died.

Carbon-12, 13 and 14, at an atomic level. Image credit: Press & Siever, retrieved from Northwestern geologist Seth Stein.

This can only take you back around a hundred thousand years or so before the carbon-14 content gets too low to be effective. But there’s another form of carbon we don’t talk about in the same breath: carbon-13, which, like carbon-12, is stable, and which is about 1.1% as abundant as the other forms of carbon.

Living organisms — as far as we’ve been able to biologically observe — seem to prefer to uptake carbon-12 to carbon-13, due to metabolic enzymes reacting with carbon-12 more efficiently. If you find an ancient source of carbon and it’s enhanced with carbon-12 as opposed to carbon-13, that’s a good indicator that it’s the remnants of an organic life-form. By looking for graphite, a form of pure carbon, deposited in otherwise highly metamorphosed rocks (things like zircons), we’ve been able to push back well beyond that 1–2 billion year barrier, and had placed the emergence of Earth-life all the way back to 3.8 billion years ago, or just some 750 million years after Earth formed. But as of 2015, we’ve done even better.

Graphite deposits found in Zircon, some of the oldest pieces of evidence for carbon-based life on Earth. Image credit: E A Bell et al, Proc. Natl. Acad. Sci. USA, 2015, via

By finding graphite deposits in zircons that are 4.1 billion years old, graphite deposits that show this carbon-12 enhancement, we now have evidence that life on Earth goes back at least 90% of Earth’s history, and possibly even longer! After all, finding the remnants of organic matter in a certain location means the organic matter is at least as old as the location it’s buried in, but it could still be even older. This is so early that it might make you think that perhaps this life didn’t originate here on Earth, but that Earth was born with life. And this could really, truly be the case.

Comet Lovejoy, photographed over Earth’s limb from the ISS in 2011. Image credit: NASA’s Earth Observatory / Dan Burbank.

The hypothesis is known as panspermia, and while there are crazy people out there who’ve taken this idea and run with it (you can find all sorts of insane rantings on the internet about it), there is some legitimacy behind the idea. You see, the Earth was not the first thing to form, but came about after legitimately over nine billion years of cosmic evolution. The entities that give rise to our planet were previous generations of stars that ended their lives in planetary nebulae, supernova remnants, and even neutron star-neutron star mergers, all of which sent heavy elements back out into the Universe.

Supernova remnants (L) and planetary nebulae (R) are both ways for stars to recycle their burned, heavy elements back into the interstellar medium and the next generation of stars and planets. Images credit: ESO / Very Large Telescope / FORS instrument & team (L); NASA, ESA, C.R. O’Dell (Vanderbilt), and D. Thompson (Large Binocular Telescope) (R).

In many cases, those heavy elements were bound together in tremendously interesting molecular configurations, configurations that we see today as truly “organic” matter.

Organic molecules are found in interstellar space in many varieties. Image credit: NASA / JPL-Caltech / T. Pyle; Spitzer Space Telescope.

When meteorites land on Earth, like the Murchison meteorite, shown below, we can analyze what’s present inside. Yes, we find all sorts of interesting organic molecules, but what’s perhaps most interesting is the amino acid content. While there are only about 20 amino acids that play a role in life processes here on Earth, there are nearly 100 unique amino acids found in this meteorite, a strong indication that the ingredients for life are ubiquitous throughout the Universe. We even find amino acids on the Moon, indicating that whatever brought these ingredients to Earth did so before the formation of the Moon, less than 100 million years into the age of our Solar System!

Scores of amino acids not found in nature are found in the Murchison Meteorite, which fell to Earth in Australia in the 20th century. Image credit: Wikimedia Commons user Basilicofresco, of the Murchison meteorite at the The National Museum of Natural History (Washington).

Well, if the ingredients are there, why couldn’t some primitive form of life be there as well? If all life on Earth has a universal common ancestor, couldn’t it be that there are many forms of ultra-primitive life in the Universe, and the type that came to Earth that was best adapted to the early Earth’s environment was the type that thrived, evolved, reproduced, and out-competed all the others? We don’t have enough evidence to favor this hypothesis over any other, but if we continue to push this limit back earlier and earlier: 4.3 billion years, 4.4 billion years, 4.45 billion years… it’s going to be harder and harder to argue that this life didn’t come to Earth already alive in some sense.

Enceladus (L), Pluto (center), and Triton (R) are all possibilities for life beyond Earth in our Solar System. Images credit: NASA / JPL / SSI (L), of Enceladus; NASA / New Horizons (mid), of Pluto; NASA / Voyager 2, of Triton (R), courtesy A. Tayfun Oner.

It’s possible that the geysers of Enceladus, the “black smokers” on Neptune’s moon Triton or even the snow-and-ice features of Pluto contain these primitive forms of life, and that it was bombardment by comets and other Kuiper Belt objects that brought an early, primitive form of life here to us. The best part about a hypothesis like this is that we can test it today, if we decide to send a mission (even an uncrewed mission) out to these worlds and check it out.

The early Solar System was filled with comets, asteroids, and small clumps of matter that struck practically every world around. Illustration credit: NASA, of the late heavy bombardment.

That’s the beauty of science: if you have an idea, all you need to do is test it out, and then you know. When it comes to the origin of life on this world — and the possible implication that it’s everywhere — wouldn’t you want to know the truth?

This post first appeared at Forbes, and is brought to you ad-free by our Patreon supporters. Comment on our forum, & buy our first book: Beyond The Galaxy!

Next Story — How certain are we of the Universe’s ‘Big Freeze’ fate?
Currently Reading - How certain are we of the Universe’s ‘Big Freeze’ fate?

The four possible fates of the Universe with only matter, radiation, curvature and a cosmological constant allowed. Image credit: E. Siegel, from his book, Beyond The Galaxy.

How certain are we of the Universe’s ‘Big Freeze’ fate?

Dark energy tells us what the Universe is doing right now, and it’s disconcerting. But its fate has some incredible possibilities.

“Even if I stumble on to the absolute truth of any aspect of the universe, I will not realise my luck and instead will spend my life trying to find flaws in this understanding — such is the role of a scientist.” -Brian Schmidt

Ever since the expanding Universe was first discovered by Hubble himself, one of the greatest existential questions of all — what will the fate of the Universe be? — suddenly leaped from the realm of poets, philosophers and theologians into the realm of science. Rather than an unknown mystery for human mental gymnastics, it became a question that the acquisition of data and a knowledge of what existed and was observable could answer. The discovery that the Universe was full of galaxies, that it was expanding and that the expansion rate could be measured, both today and in the past, meant that we could use our best scientific theories to accurately predict how the Universe would behave in the future. And for decades, we weren’t sure what the answer would be.

The star in the great Andromeda Nebula that changed our view of the Universe forever, as imaged first by Edwin Hubble in 1923 and then by the Hubble Space Telescope nearly 90 years later. Image credit: NASA, ESA and Z. Levay (STScI) (for the illustration); NASA, ESA and the Hubble Heritage Team (STScI/AURA) (for the image).

A number of astronomers and physicists were detractors of cosmology (the study of the Universe), deriding it as a science, claiming that it was merely “a search for two parameters.” Those parameters were the Hubble constant, or the present rate of expansion, and the so-called deceleration parameter, which measured how the Hubble rate was changing over time. But if the physics of General Relativity was correct, those two things would be everything we needed to know to understand the Universe’s fate. The more distant you can observe an object, the farther back in time you look. And in an expanding Universe, when you see the Universe at a younger time, not only are galaxies closer together, but they’re moving apart from one another at a faster rate! In other words, the Hubble “constant” isn’t really a constant, but is decreasing over time.

In the distant past, the Universe expanded at a much greater rate, and is now expanding more slowly than it ever has before. The best map of the CMB and the best constraints on dark energy from it. Images credit: NASA / CXC / M. Weiss.

But how it decreases over time is dependent on all the different types of energy present in the Universe. Radiation (like photons) behave differently from neutrinos, which behave differently from matter, which behaves differently from cosmic strings, domain walls, a cosmological constant or some other form of dark energy. Normal matter is simply conserved mass, so as the volume of space increases (as the scale of the Universe, a, cubed), the matter density drops as 1/. The wavelength of radiation stretches as well, so its density drops as 1/a⁴. Neutrinos first act like radiation (a-4) and then like matter (1/) once the Universe cools past a certain point. And cosmic strings (1/), domain walls (1/) and a cosmological constant (1/a⁰) all evolve according to their own physical specifications.

How matter (top), radiation (middle), and a cosmological constant (bottom) all evolve with time in an expanding Universe. Image credit: E. Siegel, from his book, Beyond the Galaxy.

If you know what the Universe is made up of at any given moment, however, and you know how fast it’s expanding at that moment, you can determine — thanks to physics — how the Universe will evolve in the future. And that extends, if you like, into the future arbitrarily far, limited only by the accuracy of your measurements. Given the best data from Planck (the CMB), from the Sloan Digital Sky Survey (for Baryon Acoustic Oscillations/Large-scale structure), and from type Ia supernovae (our most distant “distance indicator”), we’ve determined that our Universe is:

  • 68% dark energy, consistent with a cosmological constant,
  • 27% dark matter,
  • 4.9% normal matter,
  • 0.1% neutrinos,
  • and 0.01% photons,

for a total of 100% (within the measurement errors) and with an expansion rate today of 67 km/s/Mpc.

The best map of the CMB and the best constraints on dark energy from it. Images credit: ESA & the Planck Collaboration (top); P. A. R. Ade et al., 2014, A&A (bottom).

If this is 100% accurate, with no further changes, it means that the Hubble rate will continue to drop, asymptoting somewhere around a value of ~45 km/s/Mpc, but never dropping below it. The reason it never drops to zero is because of dark energy: the energy inherent to space itself. As space expands, the matter and other entities within it may get more dilute, but the energy density of dark energy remains the same. This means that an object that’s 10 Mpc away in the future will recede at 450 km/s; millions of years later, when it’s 20 Mpc away, it recedes at 900 km/s; later on it will be 100 Mpc away and receding at 4,500 km/s; by time it’s 6,666 Mpc away it recedes at 300,000 km/s (or the speed of light), and it moves away faster and faster without fail. In the end, everything that’s not already gravitationally bound to us will expand beyond our reach. In fact, 97% of the galaxies in the Universe are already gone, as even at the speed of light we’d never reach them, even if we left today.

The observable (yellow) and reachable (magenta) portions of the Universe. Image credit: E. Siegel, based on work by Wikimedia Commons users Azcolvin 429 and Frédéric MICHEL.

But dark energy may not be truly a constant. We might have measured that it evolves as 1/a⁰ according to our best measurements, but realistically, the best we can say is that it evolves as 1/a^(0±0.08), where there’s a little bit of wiggle room in the exponent. Moreover, it could change over time, where dark energy could become more positive, more negative, or could even reverse its sign. If we wanted to be honest about what dark energy can and cannot be, it’s more accurate to showcase that wiggle room as well.

The blue “shading” represent the possible uncertainties in how the dark energy density was/will be different in the past and future. The data points to a true cosmological “constant,” but other possibilities are still allowed. Image credit: Quantum Stories.

In the end, all we can go off of is what we’ve measured, and admit that the possibilities of what’s uncertain could go in any number of directions. Dark energy appears consistent with a cosmological constant, and there’s no reason to doubt this simplest of models in describing it. But if dark energy gets stronger over time, or if that exponent turns out to be a positive number (even if it’s a small positive number), our Universe might end in a Big Rip instead, where the fabric of space gets torn apart. It’s possible that dark energy may change over time and reverse sign, leading to a Big Crunch instead. Or it’s possible that dark energy may increase in strength and undergo a phase transition, giving rise to a Big Bang once again, and restarting our “cyclical” Universe.

The different ways dark energy could evolve into the future. Remaining constant or increasing in strength (into a Big Rip) could potentially rejuvenate the Universe. Image credit: NASA/CXC/M.Weiss.

The smart money is on the Big Freeze, since nothing about the data indicates otherwise. But when it comes to the Universe, remember the golden rule: anything that hasn’t been ruled out is physically possible. And we owe it to ourselves to keep our mind open to all possibilities.

This post first appeared at Forbes, and is brought to you ad-free by our Patreon supporters. Comment on our forum, & buy our first book: Beyond The Galaxy!

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