The origins of the universe

Right now we do not understand the vast majority of stuff that is out in space. Much of it seems to be invisible, and it is not part of our current understanding of physics. We also do not know how the cosmos began: what made it start growing? What happened at the very beginning? Alongside just the goal of trying to understand its life and its extent, we are still left with some big unknowns.

The idea is it is a growing universe; space has been growing. It started off in a very hot state without any galaxies or stars, just particles. It started expanding, and over millions of years galaxies began to form, and within them stars. Those galaxies merged together over the next billions of years, and one of those galaxies ended up being our Milky Way galaxy, our home. Inside there, over time, the solar system formed, which is our nearest home. We can sort of follow that. We think we have this picture of the universe growing. It follows very simple laws of physics, follows Einstein's gravity, and it has rather simple contents — simple, yet not fully understood. We think we have a good estimate that about 5% of the universe, in terms of mass or energy, is the regular atoms and particles that we know of, but again, the remaining 95% is not fully understood. We can account for it as invisible matter, called cold dark matter, and something that we often refer to as vacuum energy or dark energy that seems to be aiding the expansion of space. What is really remarkable to us as cosmologists and physicists is that we can put into our computers, into our theoretical models, a very simple initial setup of the universe 14 billion years ago, put in those ingredients and let it evolve, and the predictions of what the universe should look like today really match very well what we observe. It is pretty remarkable that we have a rather simple model that does actually match nearly all of the data.

Einstein's gravity was a refinement or an enhancement of our understanding; it told us how space responds to having massive stuff in it. If you have a huge object in space, it deforms space and makes other objects move in a way that follows that deformed space. Einstein's general relativity, his law of gravity, is the underpinning law that has the biggest impact on the large-scale universe.

Until about 100 years ago, astronomers did not know that there was anything outside our Milky Way or indeed that the universe was really changing with time, and that transition of understanding transformed the way we think about the universe and do cosmology. Since then the advances have been both observational and theoretical. We also build on an enormous amount of observations; telescopes have improved our ability to see different kinds of light that we cannot see with our eyes, and we can now see the universe in different ways. At the same time, people have developed a theoretical understanding of the universe. Over the last 50 years, that theoretical underpinning has really guided us. We are part of a long effort and a big team effort in this field.

Until quite recently, that was mostly light — it was the only thing we could pick up. Now that has changed a little; there are also other signals like gravitational waves — but let's go back to light. Your ability to do astronomy is limited by how well you can measure light from distant objects. You want to do two things: pick up a very small, subtle signal (for example, if you are looking at a distant galaxy that you cannot see with the naked eye, you want a telescope with sensitive detectors, a sensitive camera), and you want to see something in high definition. For something very tiny, you want a telescope with a big collecting dish, a large diameter to collect the light and enhance its resolution. The largest telescopes have things like eight-meter-diameter mirrors; those are in Hawaii and Chile. You also want to be in a place on Earth where the air is very still, so you can see through the atmosphere to the heavens, or you want to be in space on a space telescope.

The ability to look in many wavelengths has been a huge advance, too. Our eyes can see visible light, but that is only a tiny portion of the whole electromagnetic spectrum — the whole array of light that is out there — spanning from radio waves through microwaves, visible light, infrared light, and then ultraviolet light, gamma rays and X-rays. We now have telescopes that can look in all those different wavelengths and see this richness that you just could not see with an optical telescope or with your eyes.

Something began the expansion of the universe, and if we could go back to that time, we do not think there were any particles yet — certainly no stars, galaxies or planets, and not even any of the particles we know. Some current, popular ideas suggest that perhaps there was just a field of energy back then, often called the inflaton field, something whose potential energy drove a very rapid initial expansion. I will say that we have not definitively shown that this is true, but it is probably the most popular idea.

It would have been an almost uniform and quite condensed sea of particles and rays of light, but it would not have been completely uniform. This is critical for our own existence: there would have been slight irregularities in the density of space, and we think those were potentially put in as quantum fluctuations during the initial inflationary expansion, if indeed this is what happened. We find ourselves with this almost uniform sea of particles and light, with tiny overdense and underdense regions, and those are the conditions for the beginning of the growth of everything that comes later.

We think for about 400,000 years we had this plasma, basically an ionized plasma of particles. If a ray of light is around in that universe at that time, it will scatter off the electrons in that plasma, and a ray of light will travel for a bit, scatter off an electron and change direction. It will keep changing directions, and that behavior makes the early universe opaque, a little like a cloud. You cannot see through a cloud because you want the light to travel in a straight path to your eyes or through a medium to see it clearly.

They have been traveling through space since then. For 14 billion years of history, they have been traveling through space, and these are the ones we can capture now. They bring us an image of what the universe was like back then, and we capture this particular moment in time, back at that point in the universe, and it is the very earliest time that we can see.

The image looks like an oval, but it is really an image of the sphere around us that is just unwrapped onto an oval for viewing on the screen. The blue and red blobs, or spots, capture the intensity of the light coming from that time in the early universe's history. Those variations look quite enhanced — you see strong red and strong blue spots — but they are actually subtle variations around an average intensity. On average, this light that reaches us now has the same intensity everywhere, and it has been stretched into the millimeter wavelength. Its effective temperature is –270 degrees; it is three degrees above absolute zero. It is very, very faint, and the variations are on the order of one part in a million around that.

When we see those blobs, we are enhancing the hills and valleys of the intensity of the light at that time, and what they are tracking is the density of that sea of particles at that time in history. What I find amazing is that this density, these hills and valleys of hydrogen and helium, is the very beginning of what makes us much later. Where you have a blob in that image, you have an overdense region; gravity will simply do its job over millions of years, gathering more hydrogen and helium together in different parts of space until the very first galaxies form, and inside those, the very first stars. Only through that do you then get much later us. The image looks like spots or blobs, but they are the seeds of cosmic structure that will later make it possible for us to be here.

I feel quite comfortable thinking about millions or billions of years, because I am thinking of its whole life and then breaking it down into chunks of its life. That is a very different scale of time from the one we live in daily. Trying not to think of it all at once and thinking about it on different scales helps me, and it helps me in thinking about the scales of space, too. Rather than trying to think about how tiny we are, I think about the scale of our solar system separate from the scale of our galaxy, separate from the scales of the bigger universe, and that certainly helps me put it all together.

It is harder to visualize, but we feel that every point in the universe is part of that growth, and the growth is happening from every point. At the same time, that reminds us that nowhere is special — nowhere is at the beginning or middle of it; it has been going from everywhere. That is the thing I try to get across to push against the idea that it was some central explosion.

Editor’s note: This article has been faithfully transcribed from the original interview filmed with the author, and carefully edited and proofread. Edit date: 2025