Why do we think the Big Bang happened?

…and you may ask yourself, “Well, how did I get here?”
– Talking Heads

Everyone has at least heard about the Big Bang theory (the scientific theory, not the television show). It’s the best theory to explain the creation of the universe. But what’s less apparent to nonscientists is why we believe the Big Bang theory is correct as opposed to, say, the theory that the Universe was sneezed out of the nose of a being called the Great Green Arkleseizure?

It turns out that Big Bang theory makes predictions that can be tested observationally. If the observations confirm the predictions, the theory is upheld. If they don’t, the theory has to be either substantially modified or discarded altogether.

Cosmologist Mario Livio has an elegant writeup on why we believe the Big Bang theory is the best explanation we have for the beginning of the Universe. I’ll offer a summary here, but Mario’s post is well worth a read. First, a handy-dandy info-graphic:

Illustration of the origin of the universe

Like I said, the Big Bang theory makes some predictions that can be tested observationally. Here are the top 3:

Expansion of the Universe

Every galaxy we look at is moving away from every other galaxy*. But it isn’t because all galaxies are moving through space away from one another, but that space itself is expanding, carrying the galaxies along with it.

But how long ago was this? It turns out the expansion velocity is proportional to the distance. So for example, a galaxy that is 20 million light years from earth recedes twice as fast as a galaxy that is only 10 million light-years from earth.

Not only does this mean that the universe is expanding (and therefore was much smaller in the past than it is today) but we can also work out just how long the expansion has been going on – about 13.7 billion years.

Cosmic Microwave “Afterglow”

If there was a Bang, there should be an afterglow somewhere, right? There certainly should, but because the Universe has had time to cool down quite a bit, the afterglow should be very cool – about 2.7 kelvin cool. Sure enough, that very afterglow was discovered by accident in 1965. We know this today as the Cosmic Microwave Background and it’s temperature has been measured very precisely to 2.73 kelvin.

The Abundance of Helium

Hydrogen is the simplest element in the universe, helium is the next-simplest, formed by the fusion of two hydrogen atoms. If helium were only formed in the cores of stars, there wouldn’t be very much of it around – only about 1-2% of the elements in the universe. That’s because in such a scenario, helium could only be formed in the cores of stars, and then further fused into heavier elements such as carbon, neon, oxygen, and so on. In such a universe, helium would be fairly rare.

But our universe is about a quarter helium, so where did it all come from? It turns out the Big Bang theory makes a prediction about the conditions in the early Universe. In the first few minutes after the Big Bang, the universe would have been small enough and hot enough to fuse at least 23% of the available hydrogen into helium. In other words, the entire universe was a nuclear furnace at the time!

Other theories?

Of course, there are other Big Bang-less cosmologies, but they have a pretty high bar to clear given how well the observations match the predictions made by the Big Bang theory, and so many cosmologists remain skeptical until sufficient evidence can be produced.

Anyway, my “summary” of Mario’s post is probably a bit longer than his actual post, but I highly recommend reading it anyway. And given that we think the Universe began in a Big Bang, how might it end? Will the Universe expand forever? Will it contract in on itself in a Big Crunch?

Or will it be wiped away in the Coming of the Great White Handkerchief?

* Obviously, there are plenty of examples of galaxies that are clearly moving toward and even colliding with each other, but this is because those galaxies are so close their mutual gravitational pull overcomes the universe’s local expansion, allowing the galaxies to collide.

The deepest view of the universe: the Hubble eXtreme Deep Field

How deep into the universe have we looked? As of today, this deep:

The Hubble eXtreme Deep Field – Credit: NASAESA, G. Illingworth, D. Magee, and P. Oesch (University of California, Santa Cruz), R. Bouwens (Leiden University), and the HUDF09 Team

This is the Hubble eXtreme Deep Field, (XDF), and it’s a masterwork ten years in the making*. What you’re seeing is what you get when you take a very long exposure with two of Hubble’s best cameras of a region of the sky that contains no known stars – an ocean of 5,000 galaxies! And it’s a very deep ocean, indeed. More than 5,500 galaxies are crammed into a field of view just a fraction of the size of the full moon.

The galaxies are arranged at varying distances from us. Some are relatively bright and even have spiral arms as seen in nearby spiral and elliptical galaxies:

Nearby galaxies in the XDF resemble modern-day spiral and elliptical-shaped galaxies.

But others, way, way, waaaay in the background, don’t appear to have any structure at all. Instead, they just look like little blobs of stars and gas:

A portion of the HUDF. The tiny points of light are primordial clumps of newly formed stars, gas, and dust that would combine to form modern-day galaxies.

So what’s going on here? It turns out that these fainter galaxies are so far away, their light took billions of years to reach us. In other words, we’re seeing these galaxies as they were several billion years ago when the universe was only a few hundred million years old!

To put that into perspective, it helps to think of the XDF as a kind of “core sample” of the cosmos; the deeper into the field we look, the farther back into the universe’s past we can probe:

The XDF, separated by the distances of objects within it. The most distant objects within the XDF are more than 95% of the way back to the Big Bang.

Our universe is 13.7 billion years old. Thanks to Hubble, we can see what galaxies looked like in the current era, what they looked like in its earlier years, and what they looked like a relatively short time after the Big Bang.

And so, in just one image, we can trace the evolution of galaxies over time – from small embryonic building blocks of fluff to beautiful spirals, to giant ellipticals that are the relic of collisions of multiple galaxies. It’s the story of the universe, writ in a single image.

I’ll never tire of looking at this image, and marveling at just how far we’ve come in our understanding of the universe in so short a time.

But what really gives me goosebumps is what’s left to discover.

* I realize in retrospect I didn’t explain this elsewhere in the post. XDF is actually part of the Hubble Ultra Deep Field, which was made with Hubble’s Advanced Camera for Surveys (ACS) from September 2003 through January 2004. But this new image was made with additional ACS images taken since then, as well as Hubble’s new Wide Field Camera 3 (WFC3) which was installed in 2009. WFC3 is sensitive to near-infrared, allowing even fainter, more distant proto galaxies to be imaged. Hence my comments about this image being ten years in the making, as well as the deepest view ever!

The Milky Way’s hot halo

It’s easy to think about our Milky Way galaxy all by itself out in space, surrounded by a halo of globular star clusters, some small satellite galaxies, and that nothing else except for its neighbors millions of light-years away.

But the reality is likely quite different. Our home galaxy may be surrounded by an extended halo more than three times he diameter of our Milky Way, like this:

Artist’s impression of the extended halo surrounding our galaxy. Click to enhalonate!

That’s our home galaxy at the center, and those two puffs to the lower left are the Large and Small Magellanic Clouds, which are two irregular-shaped satellite galaxies that orbit the Milky Way. But surrounding all three is a halo of hot gas that extends for hundreds of thousands of light years, and as hot as 1-2 million kelvin!

How they do that?

A team of astronomers used NASA’s Chandra X-Ray Observatory to observe eight bright X-Ray sources hundreds of millions of light-years away; much farther away than the extent of our own Local Group of galaxies. It turns out that the some of the X-Rays from these distant sources were absorbed by by ionized oxygen gas surrounding our galaxy.

The fact that the oxygen is ionized means that the gas itself must be very hot – between 1 million and 2.5 million kelvin. And the fact that this absorption is the same no matter which distant X-Ray source we look at means there is quite a lot of this gas surrounding the Milky Way. How much? Perhaps as much as 10 billion to 60 billion suns worth. That’s a lot of hot gas!

The hot halo (not to be confused with the warm halo)

Astronomers already understand there is a halo of cooler (anywhere from 100,000 to 1 million kelvin) gas surrounding the Milky Way. But these observations seem to imply there is a lot more hot gas making up a much larger halo, indeed.

Missing baryons?

Now to be clear, these results are not yet unconfirmed. It could be that no such halo exists, and the observations can be explained by some other phenomenon. That’s ok, that’s how science works. But if it turns out that the halo is real, it could help explain the Milky Way’s “missing baryon” problem.

Baryons are ordinary matter – things like protons, neutrons, elections – in other words, ordinary everyday matter. When we look at very distant galaxies, we’re looking back in time to see how they looked when the universe was one-sixth its current age. But when we look around in our own Milky Way and nearby galaxies, it turns out there is only about half as much of this baryonic matter visible.

This halo, if its real, would contain about enough mass to account for the “missing baryons.” They wouldn’t be missing at all, they’d just be in a very extended and diffuse cloud surrounding the galaxy!

And that’s a good thing, because the universe is mostly filled with weird and mysterious stuff like Dark Matter and Dark Energy as it is. Its nice to be able to account for some of it once in a while.

First light in the hunt for Dark Energy

Dark Energy is a bit of a problem in Cosmology. It makes up 75% of the universe, it’s speeding up the expansion of the universe, and we don’t have any idea what it is.

In order to get a handle on what Dark Energy really is, we need to get a detailed survey of phenomena such as:

…over a wide swath of the sky at an unprecedented level of detail. The Dark Energy Camera, built at Fermilab, is designed to do just that. Armed with 62 CCDs, the camera takes images at a whopping 572 megapixels! When mated to the large 4-meter Blanco telescope at the Cerro Tololo Inter-American Observatory (CTIO) in Chile, the camera becomes an ultra high-resolution sky-grabber:
Small Megellanic Cloud imaged by the Dark Energy Camera

That’s the Small Magellanic Cloud – a small satellite galaxy to our own Milky Way about 200,000 light years away. Of course, the SMC has been imaged in its entirety before, but not to such a high resolution in a single image.

Over the next five years, the camera will create detailed images of one-eighth of the southern sky. One-eighth might not seem like a whole lot, but that’s 5,000 square degrees – enough to discover 300 million galaxies, measure 100,000 galaxy clusters, and detect 4,000 supernovae. Suffice to say, that is a lot of data, and astronomers are going to need it if they are going to get a better handle on what Dark Energy is. After all, if we cannot see it, we have to look at the universe on a very large scale if we’re going to be able to measure its effects on the stuff we can see.

So, the Dark Energy Camera is locked and loaded at the prime focus of the Blanco telescope. just about ready to begin its investigation into something that makes up the overwhelming majority of the universe that we didn’t even know about until 15 years ago. I can’t wait to see what we learn.

Credit: Dark Energy Survey Collaboration.

Magnifying the Universe

Magnifying the Universe poster
Excerpt from the Magnifying the Universe poster. Click this, it’s cool.

“Space,” it says, “is big. Really big. You just won’t believe how vastly, hugely, mindbogglingly big it is. I mean, you may think it’s a long way down the road to the chemist’s, but that’s just peanuts to space, listen…” The Hitchhiker’s Guide to the Galaxy, by Douglas Adams

Now that we’re up, up and away, a little orientation of the universe seemed like as good a way as any to get started. The problem is that  the sheer scales of even the most trivial objects in the universe are so beyond our day-to-day experiences is that it is very difficult to get our heads around them.

That shouldn’t surprise us. After all, we evolved to survive on a planet that happened to be of a certain size. Perhaps if we were some sort of interstellar whales we might have a more intuitive sense of the sizes and distances of everything from small asteroids to the largest stars. But even then we might have a hard time contemplating the relative sizes of galaxies and the vast distances between them.

Fortunately, such scales are not beyond our mathematics, nor beyond our ability to express them in a convenient infographic courtesy of the folks at numbersleuth.org. Be sure to click for the full size as it’s worth a fun little read.


While I trust that their scales check out, some of the imagery used is incorrect or misleading. For example, the image of the solar system chosen only goes out to the orbit of Jupiter. It’s much larger than that, of course. The first two images of the poster for the Observable Universe and the Observed Universe should be switched and labeled differently. Still, it’s a pretty fun way to get a handle on the scales of these things.

But wait, there’s more!

As an added bonus, the same folks were kind enough to organize the infographic into a handy zoomable interactive:

Copyright 2012. Magnifying the Universe by Number Sleuth.

Enjoy zooming around!