What’s in the Universe?

“There is only one corner of the universe you can be certain of improving, and that’s your own self.” –Aldous Huxley

Earlier this week, I told you the story of how we went from a Universe that was — at one time — almost perfectly smooth, full of tiny, random fluctuations in density,

to the Universe we have today, full of stars, galaxies, and clusters of galaxies all clumped together in a beautiful cosmic web of structure.

But there was one picture I showed that generated a lot of questions. I put up an image showing what the Universe was made out of today (when we have this great cosmic web), and what the Universe was made out of back when the cosmic microwave background (the tiny, one-in-a-hundred-thousand density fluctuations) was emitted. Not surprisingly, the two pie charts look very different.

But this is one of the most beautiful things about not just astronomy, but astrophysics.

If you give me a theory of gravity and how it relates to space and time, like Einstein did nearly 100 years ago,

and an expanding Universe that’s roughly smooth on large scales, then all I need to know is what’s in the Universe at any one time (like say, today), and I can tell you a whole slew of things!

Like what, you ask?

  • The energy density of each component of the Universe at any time,
  • The expansion rate of the Universe at any time,
  • The age of the Universe,
  • The size of the observable Universe, and
  • How important any part of the Universe is — atoms, dark matter, dark energy — at any given time.

The equations that relate what’s in the Universe to the size, age, and expansion rate of the Universe have been well-known since the 1920s, and were the basis for Hubble’s conclusion that the Universe was expanding.

But fast-forward to today, where we know how much of today’s Universe is made out of dark matter, dark energy, atoms, neutrinos, and photons. For the first time in history, actually, we know these numbers accurately: to within about 1%.

So I can do the math, and make some graphs for you. The observable Universe is presently 93 billion light-years in diameter, give-or-take about 1%. But, since it’s expanding, it was smaller in the past. If we go back to when it was 10%, or 1%, or 0.1% of its current size, how important were each of the five things — dark energy, dark matter, normal matter (atoms), photons, and neutrinos — we know our Universe is made out of?

As always, click for the full-size version.

Well, the density of dark energy — the densest thing right now — was the same in the past. (That’s why it’s sometimes called a cosmological constant.) But both normal matter and dark matter were even denser in the past! When the Universe was 10% of its present size, or 9.3 billion light years in diameter, dark matter and normal matter were the most important things in there.

But go back even farther, and you’ll discover that photons, which started out today with the lowest energy density (at about 0.008%), overtake not only neutrinos and dark energy, but even normal matter and dark matter at early enough times!

Why’s this?

Because when your Universe was smaller — in the past — the wavelength of your photons was also shorter, which means they had more energy! Anything that acts like radiation — including neutrinos at these early stages — loses energy at a faster rate as the Universe expands than matter of any type — whether dark or normal — does.

So if we put all of this together, and say that the total energy of the Universe at any time is 100%, what do we find that the Universe is made out of at any given time?

This. Today, dark energy is just starting to take over as the most important form of energy in the Universe, from dark matter. It happened, in fact, when the Universe was already about 2/3 of its current size! Before that, matter was king, with dark matter at about 83% and normal matter at around 17% of the Universe. And this was true from when the Universe was about 1% of its current size (when it was only about 10 million years old) until dark energy took over.

But when the Universe was even smaller? Radiation — in the form of photons and (to a slightly lesser extent) neutrinos — was not only important, but at small enough scales, dominated the Universe. Radiation dominates all the way back until inflation happened, and our Universe, as we know it, first began.

The cosmic microwave background came at an important transitional time in the Universe, when it was about 0.092% of its current size. At that time — when the Universe was only 380,000 years old — neutrinos, photons, normal matter, and dark matter were all energetically important components of the Universe!

And that’s why the Universe looks so different — in terms of energy — when it was so much younger! And now you know what’s in the Universe at any given time. Want to know what it’ll be like farther into the future? Just extend these graphs I made, and you’ll find that dark energy takes over, leading to a cold, bleak, lonely future.

How lucky are we to be here right now?