Behind the Curtains of the Cosmos 3: Keys to the Cosmos

by Shane L. Larson

The fact that the curtain of the Cosmic Microwave Background exists is a huge boon to astronomers, giving us confidence in our understanding of Big Bang cosmology. But is that all? We have all this microwave light coming at us from every direction in the Cosmos — what can we do with it?  Is there anything else we can learn?

As it turns out, there is more to the light than meets the eye. Astronomers excel at taking information from the Cosmos and digging deeper, to tease out secrets and information that at first glance are not easy to see.  If you look closely at the Cosmic Microwave Background you discover some Very Important Things. One thing is that it is not absolutely, perfectly smooth. Imagine picking up a ball off a pool table — it looks and feels very smooth; that is analogous to the uniform temperature of the Cosmic Microwave Background looking the same no matter where we look on the sky.  But if you take that same pool ball and look at the surface closely with a magnifying glass, you find that it has some scratches and roughness to it — they are so small as to not be detectable by your fingertips, nor to your naked eye.

(L) A billiard ball looks and feels smooth to your eye and your finger tips. (R) But if you can look at it very carefully under a microscope, you find there are minuscule imperfections that are only detectable with excellent technology.

The Cosmic Microwave Background is very similar. If we use a highly accurate microwave telescope to look, we find that there is a small scale  “roughness” to the sky; astronomers think this roughness is an indicator of clumpiness at the time atoms formed, ultimately leading to what you and I see today — galaxies and clusters of galaxies in the Universe. This roughness was first detected by the COBE satellite in 1992. It was later mapped at even finer precision by the WMAP mission, and by the Planck mission.The spots on this map can be thought of as hot and cold spots on the sky. The red spots are the hottest, at only about 2.7 Kelvin (1 Kelvin is 1 degree Celsius) above absolute zero. The blue spots are the coolest, about 0.00001 Kelvin cooler than 2.7 Kelvin — a difference almost so small as to be undetectable except with the most exquisite astronomical instruments! Before the microwave background broke free from matter, the small variations in the density of matter were spread across the Cosmos. When the matter condensed to form neutral atoms, these variations imprinted themselves in the Cosmic Microwave Background. The spotted map of the Cosmos seen by COBE, WMAP and Planck is a message from the other side of the curtain!

The Planck All-Sky Map of the Cosmic Microwave Background variations. [Planck Collaboration]

Is there any other property of the Cosmic Microwave Background that might be measured? Is there any other physical imprint that might tell us something about the early days of the Cosmos?  Of course there is!  It is called polarization. Light is known to scientists by its proper name, “electromagnetic radiation.” As the name suggests, it has an electric part and a magnetic part, called “fields.” The way we visualize light is as a wave of electric fields and magnetic fields travelling together, at right angles to each other. For any light you receive, the direction the electric field is waving defines the polarization of the wave.

Light is a propagating “electromagnetic field,” a waving electric field travelling together with a waving magnetic field. The direction the electric field waves defines the polarization of the light.

Polarization encodes many kinds of information, related to how light and matter interact with one another. The most common way you encounter this every day is with sunglasses. Reflected light is polarized, so we build sunglasses that block polarized light, cutting down glare off of reflected surfaces. You might have noticed that the display of your smartphone is polarized too — you can’t see it in certain orientations through your polarized sunglasses.

Light reflected off of surfaces is polarized, like these mud flats. (L) The waves and the sand look bright when polarized light is seen. (R) When polarized light is blocked, as with polarized sunglasses, the appearance changes dramatically. [Image from Wikimedia Commons]

Light from the Cosmic Microwave Background is expected to be polarized, imprinted with patterns because during decoupling (the time when electrons were binding to nuclei to form atoms) the light bounced off of the free electrons (in physics-speak: it “scattered”) giving it a definite polarization. Astronomers call these polarization patterns “E-modes” and they are recognizable because they make symmetric patterns in the sky — the polarization pattern looks exactly like itself if you look at a reflection of the pattern in a mirror. This kind of pattern was discovered by the DASI experiment in 2002.

Polarization of the Cosmic Microwave Background detected in 2002 by the DASI Collaboration. These are “E-mode” polarizations, caused by scattering.

So what does this all have to do with inflation? Inflation happened shortly after the Big Bang, before anything that you and I might recognize as a particle had formed. The forces of Nature spontaneously appear as the Universe cools, enabling different kinds of physical interactions to appear. Gravity had appeared sometime before inflation, during a time cosmologists call the “Planck epoch.” Inflation was the sudden, rapid expansion of everything — the energy soup, and spacetime itself — from the incredibly tiny point that expanded to become the Observable Universe. Think of spacetime like a balled up bundle of wrapping paper being unfolded, flattened and smoothed out by inflation.  On the smallest scales, spacetime is changing — expanding, stretching, flexing of the folds in our bundle of paper. Physicists call these happenings “quantum fluctuations.” The stretching and unfolding of spacetime means the gravitational field is changing. But that’s exactly what we said causes gravitational waves! As the Universe inflates, the quantum fluctuations in spacetime itself generate gravitational waves. This has physicists really excited, because detecting these gravitational waves would be the first hint of the quantum nature of gravity itself.

As we noted last time, gravitational waves interact with matter very weakly, so they propagate through the slowly evolving soup of the Cosmos, pushing matter here and their until the formation of the Cosmic Microwave Background.  What is the net result? The net result is that gravitational waves leave a polarization imprint in the microwave light. Just as with the expected polarization from electron scattering, there is a pattern to the polarization made by the gravitational waves. Astronomers call this pattern “B-modes” — they have a twist to them.  You can recognize a B-mode pattern, a twist, because in a mirror the pattern appears reversed.

(L) E-mode polarization patterns look identical if viewed in a mirror. (R) B-mode polarization has a “twist.” If the twist is clockwise, then when viewed in a mirror the twist is counter-clockwise, and vice versa.

Which brings us back to the story of BICEP2. Astronomers have been looking for the tell-tale twist of polarization in the Cosmic Microwave Background for some time. Major experiments have been slowly gearing up to look for and characterize the unique, gravitational-wave signature of inflation. The science team at BICEP2 won the race. What has all of us so excited is the measured value is larger than we anticipated, indicating the relative importance of the gravitational waves is large.  This has provided sudden and unexpected guidance for theoretical physicists trying to model inflation, and for future experimenters attempting to build new experiments to probe the early Universe.

The BICEP2 polarization map, showing B-mode (“twist”) patterns.

So what now?  Astronomy is a spectator sport — we keep looking!  Now that our colleagues at BICEP2 have made the initial detection, we are gearing up to look more closely at the result, and to dissect it for clues that confirm we are on the right track to understanding the Cosmos.  New papers are appearing rapidly (I counted about 100 at the time of this writing). The result doesn’t precisely agree with other results we already have (there is “tension” between the results, in physics-speak), and some of us still have reserved skepticsm.  The field has been thrown into a big mess, and now we have to figure out what happened. It’s a bit like coming home to find your living room in a disastrous state, and trying to figure out if it was the kids, the cat, the dog, or some combination thereof that made the mess! But astronomers don’t mind — it’s the figuring out of the mess that is so rewarding. But make no mistake — it was an outstanding achievement, a triumph of the human intellect and human ingenuity. And onward we go.

The BICEP2 Telescope at the South Pole. [Image: National Science Foundation]

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This is the last installment in a 3-part series about the March 2014 BICEP2 announcement about the detection of a putative signal from inflation in the Cosmic Microwave Background.  Part 1 can be found here.

Behind the Curtains of the Cosmos 1: Inflation and the Microwave Background

by Shane L. Larson

The world was all atwitter last week (on twitter and otherwise) with the announcement from our friends at the BICEP2 collaboration that they had detected the echo of the Universe from the earliest moments after the Big Bang. (links to press releases and videos discussing the result can be found here).

The BICEP2 Telescope at the South Pole. [Image: National Science Foundation]

Have no doubt — this achievement is remarkable, a testament to our ability to probe and understand the secrets of the Cosmos that Nature has left for us to find. Nobel Laureate Stephen Weinberg once waxed poetic in his book The First Three Minutes, claiming, “The effort to understand the universe is one of the very few things which lifts human life a little above the level of farce and gives it some of the grace of tragedy.”  This is one of those moments; an instant in time when every member of our species should raise up their head, stand a little taller with pride and know that despite all the trials and tribulations that face our race and our planet, we are capable of great things.

But what does it all mean? What is the hub-bub all about?  There are two elements of this that you will hear people talking about.  One is about inflation and one is about gravitational waves.  Today, let’s talk about inflation.

What is inflation all about?  In order to understand inflation, let’s first talk about The Big Bang.  Most of us have heard about The Big Bang, a model for the Cosmos that arises naturally from the notion of taking what we can see today (the expansion of the Universe) and running the movie backward in time.  The development of Big Bang cosmology evolved as many ideas in science do — a few fundamental observations inspire someone to imagine what the underlying Laws of Nature might be. Those ideas are written down, and everyone begins to think of all the possible consequences — what kinds of things must be true, and how could we observe them? Big Bang cosmology is a result of imagining “running the movie backward” from the current, expanding state we see when we look out from the Earth.  If you shrink the current Observable Universe into the distant past, then it should have been smaller, and as a consequence much denser and hotter.

So what was it like, the tiniest fraction of a second after the Big Bang?  It was hot. Our understanding of the densities and temperatures then suggest it was 10³² ºC = 100,000,000,000,000,000,000,000,000,000,000 ºC. What does that even mean? A temperature like that is so far outside our common, everyday experiences with coffeepots and campfires as to seem quite ridiculous.  But to an astrophysicist, high temperatures are synonymous with great speeds.  If the Universe was this hot, then that means everything in the Universe was flying around at tremendous speeds. Since the Universe was much denser, everything was tightly packed together, increasing the number of collisions.  What happens when you have collisions at high speeds? You break things apart!

At this time, things are crashing together so hard that atoms can’t exist. In fact, things are crashing together so hard that the nuclei of atoms can’t exist.  The entire Universe is filled by a Primordial Soup of protons, neutrons, electrons, and photons (light) — energy, and the building blocks of atoms.

As the Universe expands, it cools. By 100 seconds after the Big Bang, the Universe has cooled to a mere 10 billion ºC.  At that temperature, protons and neutrons can stick together to form the cores of the first atoms. At these temperatures, when they crash together, the strong nuclear force is strong enough to keep them together. This moment of creation of the nuclei is called nucleosynthesis.

Nucleosynthesis was the formation of the cores (the nuclei) of atoms. Most were hydrogen nuclei (single protons) with a little bit of helium, and an even smaller amount of lithium. Observations of these proportions is one of the strongest pieces of data supporting Big Bang cosmology.

Now, 10 billion ºC is still too hot for electrons to bind to these newly formed nuclei; each time they try, a collision knocks them loose, so no proper atoms can form at this time.  The result is that the early Universe is a swirling maelstrom of charged particles. The consequence of that fact is that light is effectively trapped. Photons (light) are easily deflected (astronomers use the term “scattered”) by charged particles. If a photon is flying along minding its own business and happens upon a charged particle, it gets bumped off its path in a new direction. Since the early Universe is very dense, it soon encounters another charged particle and it gets scattered again! The result is that a photon’s path through the early Universe is a bit like a drunken sailor’s walk, and has little chance of ever looking like anything else.

The Universe continues in this state for about 400,000 years, continuing to expand and continuing to cool. Around 400,000 years the temperature has fallen to a balmy 3000 ºC. At that temperature, electrons can stick strongly enough to nuclei that collisions don’t knock them free.  Suddenly, neutral atoms form!  This event is called “recombination,” which always struck me as a funny term because this is the first time particles combined to form atoms! Almost all of the atoms that form are hydrogen, mixed with a good amount of helium, and a little bit of lithium. These amounts have been measured by astronomers and agree exquisitely with the amounts predicted by Big Bang Cosmology; this is one of the strongest pillars of evidence convincing us that the Big Bang is the correct model for the Universe.

(L) Before atoms form, the Universe is filled with a vast soup of charged nuclei and electrons, and photons cannot travel very far without scattering off of them. (R) When the electrons bind to nuclei (“recombination”) to form neutral atoms, the photons no longer scatter so easily, and they fly free — the Universe has become transparent.

The most important consequence of recombination is that all the charged particles are bound together into neutral atoms. As far as the photons are concerned, the Universe is suddenly devoid of any charged particles to scatter off of, and they burst free and start travelling on long, unscattered paths. This is called “decoupling” — the photons no longer strongly interact with the “stuff” that became atoms. Free to stream through a suddenly transparent Universe, the photons begin their long 14 billion year journey to us.  We see them here on Earth as microwaves, coming from every direction on the sky.  We call these microwaves the Cosmic Microwave Background, and it is the relic light from the moment that atoms formed.

Arno Penzias (L) and Robert Wilson (R) in front of the Bell Labs microwave antenna that first detected the Cosmic Microwave Background. They were awarded the 1978 Nobel Prize in Physics for their discovery.

The Cosmic Microwave Background was discovered in 1965 by Penzias and Wilson at Bell Labs. As expected, it was coming from every direction on the sky, and exceedingly uniform — it looks exactly the same, to our ability to measure it, no matter which direction we looked. The Cosmic Microwave Background is relic radiation that is a precise probe of the temperature at the time atoms form. The fact that we see the same microwaves in every direction means atoms formed at the same time in the same amounts at the same temperature in every direction.

A recreation of the 1965 Cosmic Microwave Background map, covering the entire sky (Penzias and Wilson could not see the entire sky from Bell Labs). The band of stronger microwave light is the signature of the Milky Way Galaxy.

Despite the fact that this is exactly what was predicted, it poses a certain problem for us to understand. The uniformity is confusing because the way things get to be the same temperature is through thermal contact — they talk to each other, and trade bits of energy (heat) until they are at the same temperature. This is why Popsicles melt if you leave them on your counter — heat from the counter flows into the Popsicle, melting it until the puddle of goo is the same temperature as the counter. This is why your cup of coffee cools down — heat from the coffee flows into the air, warming the air in the room until the coffee and the air are at the same temperature.  So why should that bother us with the Cosmic Microwave Background? Because there is an ultimate speed limit in the Universe — the speed of light.

To understand this, look up in the sky to your left. The microwave background in that direction is so far away it has taken the entire age of the Universe for the light to reach Earth!  Now look up in the sky to your right — the microwave background is just as far away in the other direction! In the entire age of the Universe, these two patches of the sky have only had time to send light to the Earth, NOT to each other.  But that means we have a great mystery! The two patches are at EXACTLY the same temperature as each other. How did they know to be the same temperature if they aren’t in thermal contact, if they can’t talk to each other and trade heat in less than twice the age of the Universe?   This is called “The Horizon Problem” — pieces of the sky are too far away from each other to collude to have the same appearance and physical properties.

The page from Alan Guth’s research notebook where he had the first idea for inflation (box at the top). [[On display at the Adler Planetarium; the notebook is part of the collection curated by the Adler’s Webster Institute for the History of Astronomy.]]

The solution to the problem was initially discovered by Alan Guth in 1979.  Guth imagined a moment very early in the Universe. How early? About 10^–35 seconds after the Big Bang (about a hundred-billionth of a trillionth of a trillionth of a second!). At that time, the Universe was very small, and as a result very hot and very dense. The fact that it was very small means than the entire Universe was in thermal contact — it could all trade heat until it all had the same temperature.  So how to you grow a very tiny, uniform temperature Universe into a gigantic huge Universe that still looks like it is in thermal equilibrium?  Guth realized the way to do this was to grow the Universe very large, very fast.  This sudden, rapid expansion is called inflation, and it occurred very shortly after the Big Bang, from about 10^–36 seconds to 10^–34 seconds! During that time, the Universe inflated, like a balloon, by a factor of about 10²⁶ in size.  Now 10²⁶ is clearly a big number (100 trillion trillion), but what does it mean to expand by that factor?  Imagine an atom.  An atom is about 10^–10 meters across (1 angstrom).  By contrast, a lightyear is nearly 10¹⁶ meters, or a factor 10²⁶ bigger than an atom. If you grew an atom to be a lightyear across, it would have increased in size by a factor of 10²⁶.

Cosmologists put this all together in a chain of reasoning that can be hard to keep in your head. Here are the salient points:

1. Early on, everything was extremely close together and the Universe was in thermal equilibrium — all the parts of it were in contact and reached the same temperature.
2. Inflation suddenly occurs — the Universe is stretched and pushed apart, so that different parts of it are so far away they are no longer in thermal contact.
3. The now distant parts have the same thermal properties because they were in equilibrium before the expansion!
4. Now, after inflation, the different parts of the Universe continue to cool, but they all cool in exactly the same way, so they look as if they are at the same temperature.

Voila! Horizon problem explained!  🙂

At this point, you might think we’re done because the idea of inflation neatly explains some of the observational questions we have about Big Bang cosmology. But we’re scientists, so we’re always pushing hard on our ideas to make sure they are right, to understand all that they imply.  So what else does inflation do that we can measure?  Is there anything we can observe that continues to confirm the basic principle of inflation? Are there measurements that can be made that might explain more about how inflation started, how it stopped, or whether is is something more complicated than plain old simple inflation?

Observing the Universe at the time of inflation is hard.  Why? We know inflation happened right after the Big Bang, almost 400,000 years before the creation of the Cosmic Microwave Background.  The Cosmic Microwave Background is like a curtain — it is the oldest light we can see, because it was released at the first moment when light could travel freely.  Before that moment, light was inhibited from travelling very far at all — it scattered and bounced around and could never make a beeline for Earth and our telescopes.  As a consequence, we can’t see beyond the Cosmic Microwave Background. If we have any hope of probing our ideas about inflation, we’re going to have to find a way to see beyond the Curtain, or discover some unique signature that inflation left in its wake that imprinted itself in the Cosmic Microwave Background.  Fortunately for us, these are not idle wishes.  One thing inflation should have done is generate an echoing background of gravitational waves that should have left their imprint on the Cosmic Microwave Background.  This will be our focus next time.

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This is the first in a three-part series to explore physics and astronomy behind what the BICEP2 observation of the Cosmic Microwave Background is all about. The other two parts are:

2: Gravitational Waves (5 April 2014)

3: Keys to the Cosmos (11 April 2014)