“One of the most tragic things I know about human nature is that all of us tend to put off living. We are all dreaming of some magical rose garden over the horizon instead of enjoying the roses that are blooming outside our windows today.” -Dale Carnegie
Our new Ask Ethan segment has been really popular, and the questions and suggestions keep pouring in. It’s your Universe too, and if there’s something you want to know about it, you should ask! (So keep it up!) This week’s question is one of the biggest of them all, and it comes courtesy of John L. Ferri, who asks,
I have a difficult time understanding cosmic inflation and the horizon problem. I think you covered it once before, but more detail may help my confusion.
Let’s back up to the beginning, to put this in some context.
This is your Universe. It goes on as far as the most powerful telescopes ever devised can see in all directions, finding tens of thousands of galaxies located tens of billions of light years away everywhere we’ve ever looked. On the largest scales, it’s roughly the same — the same densities, temperatures, star-and-galaxy types, metallicities, etc. — no matter where we attempt to look. The biggest difference we’ve found is that the farther away we look, the younger the things we’re looking at appear to be, and the faster they appear to be receding from us.
This has to do with how the Universe is expanding and evolving. Thanks to three big pieces of evidence in the context of General Relativity (our theory of gravity):
- The Hubble expansion of the Universe, or the observation that a galaxy’s apparent redshift correlates with its apparent distance from us,
- The existence and properties of the Cosmic Microwave Background (CMB), or an almost-perfectly-uniform sea of blackbody radiation in all directions just a few degrees above absolute zero, and
- The abundances of the light elements — hydrogen, deuterium, helium (He-3 and He-4) and lithium — in the earliest recesses of the Universe, before any stars had formed,
we can safely conclude that the Universe evolved and expanded from a hotter, denser state, and that has been around in its current matter-and-radiation-filled form for about 13.8 billion years. A long time, but not an infinite amount of time. This paradigm is known colloquially as the Big Bang.
But there’s an issue here. Over the past 13.8 billion years, the Universe has expanded according to the rules of General Relativity, which means the rate that space has expanded is determined by some set of initial conditions and also on the energy content (normal matter, dark matter, radiation, neutrinos, dark energy, spatial curvature, etc.) of the entire Universe. That part is fine, but the issue is that the Universe appears to have the same rough, average properties everywhere we look.
The densities and clustering properties of galaxies on one side of the Universe are identical to those observables on the other side, no matter what arbitrary “side” you choose. This should strike you as weird. Why?
Consider that since the Big Bang, no information has been able to travel faster than the speed of light. We can “see” 46 billion light years in each direction because that’s how far light has been able to travel in the expanding Universe over the past 13.8 billion years. So if we can look more than 23 billion light years in one direction and more than 23 billion light years in the other direction, we wouldn’t expect these regions to be related.
Still confused? Let’s give you an analogy to help understand this better.
Consider boiling water in a pot on your stove. You heat the water from the bottom, and — as best as you can tell — the water boils at the same time everywhere in the pot. There’s no discernible temperature difference between the top of the water and the bottom. Think about that fact for a second.
Why is that?
You’re heating the pot of water from the bottom, but the water is heating up everywhere. This is because the water molecules are moving around, bumping into other water molecules, and sharing their energy among one another. You may be heating the water from the bottom, but the water from the bottom can interact — and exchange information/energy — with the water in the rest of the pot, and can do it on timescales that are very small compared to the timescale of heating the water to a boil.
This isn’t always the case.
Consider the above photo, where lava flows down from an active volcano and into the ocean. Where the lava — well over 1,000 °C — strikes the water, it boils almost immediately due to the incredible temperature difference. But the ocean is vast, and the rate of heat transfer is finite; you don’t have to go very far away from where the lava enters the water at all to find waters where the temperature is virtually unaffected by the lava heating the water. For all practical purposes, those regions are causally disconnected from one another, because they do not exchange information or share properties with one another.
It would be very surprising if causally disconnected regions — regions that didn’t come in contact with one another or exchange information with one another — had the same temperatures. Yet that’s exactly what the Universe appears to do!
Consider that the Cosmic Microwave Background (CMB) was emitted when the Universe was just 380,000 years old, and that in that time — even in our rapidly expanding Universe — light could have only traveled around one million light years in any direction. If you filled in the microwave sky with circles a million light-years in radius, it would take more than ten billion independent regions to fill what we can see! And yet, these causally disconnected regions have the same temperatures, spectra, and densities to about 99.99% precision.
That problem — that regions that have never had a chance to exchange information with one another just happen to have identical properties to one another — is known as the horizon problem.
But there is a way to solve it; these regions may not have had time to exchange information with one another since the Big Bang, but what if the Big Bang began with them already having the same properties?
In a nutshell, that’s what inflation is: the thing that happened before the Big Bang that not only sets it up, it sets up the initial conditions that our Universe appears to have! By taking a small, tiny region (possibly even an infinitesimal region) and expanding it exponentially, that tiny region stretches to a size larger than our presently observable Universe, and ensures the following:
- That any matter, particles, energy or topological defects existing in that region of space prior to inflation will be so reduced in density that — at most — there will be only one such particle left in our Universe (solves the monopole problem),
- Whatever curvature space had prior to inflation, inflation stretches it so that it will appear to be indistinguishable from flat when we look at it post-inflation (solves the flatness problem),
- Whatever variations there were in temperature or density across different regions of space prior to inflation, it’s only one tiny region that gave rise to our entire observable Universe, explaining why our Universe appears to have the same temperature-and-density properties everywhere we look (solves the horizon problem), and
- Quantum fluctuations that take place (according to well-understood laws) during inflation give rise to a very particular set of predictions for temperature-and-density fluctuations (and imperfections) in our observable Universe today. (Some more explanations here, here and here.)
Item number three is how the horizon problem is solved: by taking a tiny region where things once were connected and stretching it to such a large size that everything we see — although they’re not connected since the Big Bang — was once connected before the Big Bang.
If you don’t allow inflation, then you simply have to sweep those three problems (monopole, flatness and horizon) and one prediction (about the now-confirmed spectrum of density-and-temperature fluctuations in the Universe) under the rug, and say, “Those are just the initial conditions the Big Bang started with” to make your model work.
Or, you can embrace inflation as the simple, elegant and straightforward way to solve all of them.
And that’s why the Universe — to the best of our knowledge — is the same everywhere and in all directions.
Have a question or suggestion? Drop me a line in our question/suggestion box, and your idea could get the Ask Ethan treatment next!