Tag Archives: Cosmic Microwave Background

Gravity 10: Signatures of the Big Bang

by Shane L. Larson

Science has two interlocking pieces that always work together. One part is “describing the world to predict the future and explain the past.” This is the part that many of us remember from science class, involving pen and paper and mathematics and the Laws of Nature. Another part is “observing the world and seeing what Nature is up to.” This is also a part that many of us remember from science class, involving doing experiments and recording numbers and making graphs. These two pieces are called theory and experiment respectively, and they constantly validate and reinforce each other in a never-ending cycle of upgrading and refining our knowledge of the Cosmos.

All science is a combination of theory and experiment. In Cosmology, theory is an application of general relativity embodied in the Friedmann Equations (left); experiment is captured in astronomy (right).

All science is a combination of theory and experiment. In Cosmology, theory is an application of general relativity embodied in the Friedmann Equations (left); experiment is captured in astronomy (right).

Cosmology has seldom had both theory and experiment walking hand in hand. Instead one or the other has been out in front — sometimes way out in front — waiting for the other to catch up. This was certainly the case when general relativity was announced to the world — as a description of the machinery of the Cosmos, it was perfectly capable of making predictions that were so far beyond our ability to observe and verify that we didn’t recognize the truth for what it was. This famously happened early on, in 1917 when Einstein published one of the earliest papers about relativistic cosmology entitled “Cosmological Considerations in the General Theory of Relativity.”

As city dwellers, it is often easy to forget that the sky is full of stars. This fact leads to the most natural assumptions about the Universe based on experience: the Universe is full of stars.

As city dwellers, it is often easy to forget that the sky is full of stars. This fact leads to the most natural assumptions about the Universe based on experience: the Universe is full of stars.

At that time, we were profoundly ignorant about the nature of the Universe. The prevailing view was that the Cosmos was full of stars, and that the Universe was static. It’s the most natural assumption in the world based on your experiences when you step out the door every night — the sky is full of stars and they change little, if at all, night to night.

Einstein considered a Universe simply filled with stars, and asked what general relativity predicted. He found it only predicted one thing: the Universe must collapse.  Preconceptions are a powerful force in science, and Einstein believed strongly in the static Universe, so much so that he supposed maybe he had not gotten general relativity completely correct. So he introduced a mathematical addition to general relativity that pushed back against the collapse, called the Cosmological Constant.

Before 1924, the nature of galaxies was unknown. They were grouped with the "nebulae" -- wispy, cloud like structures that could be seen through the telescope. These included Messier 31, the Andromeda Nebula (L) and Messier 51, the Whirlpool Nebula (R). The Whirlpool was the first nebulae that spiral structure was detected in, by Lord Rosse in 1845.

Before 1924, the nature of galaxies was unknown. They were grouped with the “nebulae” — wispy, cloud like structures that could be seen through the telescope. These included Messier 31, the Andromeda Nebula (L) and Messier 51, the Whirlpool Nebula (R). The Whirlpool was the first nebulae that spiral structure was detected in, by Lord Rosse in 1845.

But science is always on the move. Telescopes in that day and age were getting larger. In 1917 the 100-inch Hooker telescope first saw starlight on its mirror, as it embarked on a long and storied history of astronomical discovery. Telescopes gave us the capability to probe deeper into the Cosmos, and we had started to discover vast diaphanous complexes of light that showed no stellar qualities. These were called nebulae, Latin for “cloud.”  A few of the nebulae, the “spiral nebulae,” perplexed astronomers — some thought they were simply odd nebulae (vast complexes of gas and dust), and others thought they were island Universes (other galaxies, like the Milky Way).

The 100-inch Hooker Telescope on Mount Wilson, used to discover the expansion of the Cosmos.

The 100-inch Hooker Telescope on Mount Wilson, used to discover the expansion of the Cosmos.

The discovery of the distances to the spiral nebulae, using Leavitt’s Cepheid variable method, was a watershed moment in the history of cosmology. It put on the table two  important facts: first, the Universe was vast — enormously vast — with distances far beyond the boundaries of our own galaxy.  Second, the major constituent especially on large scales, was not stars, but galaxies (which are agglomerations of stars). These two simple facts suddenly and irrevocably changed the way we thought about the Universe. In the first years after the discovery of the nature of galaxies, Friedmann and Lemaître used general relativity to imagine a Universe filled with galaxies, and discovered the idea that the Universe was expanding. This notion, discovered on paper, was bourne out in 1929 when Humason and Hubble, once again using the 100-inch telescope on Mount Wilson, found all the galaxies in the Universe were receding from one another.

At that time, Lemaître made a great leap of imagination — it was a thought that was well outside the comfort zone of astronomers of the day, though today we may view his leap as completely obvious. From the mindset of the future, it is difficult to imagine just how hard it was to think different. Lemaître supposed that if the Universe was expanding, then in the past it would have been smaller, and hotter. The idea was met with incredulity and derision, sparking enormous debate for decades to come. But science is the blend of ideas on paper with observations of the Universe. If Lemaître’s ideas were right OR wrong, the evidence could be found by looking into the Cosmos.

There are many lines of evidence that confirm the basic idea of the Big Bang, but there are three major pillars of support. The first, is the expansion itself. The Universe is not like a sports car, starting and stopping on a whim, braking and accelerating at random. Its evolution is driven by the Laws of Nature, in a smooth and predictable fashion. If we see expansion today, that expansion started in the past. The rate and trends in the expansion are a function of the amount of matter in the Universe, and the initial conditions of the expansion. These quantities can be determined from astronomical observations, and are consistent with the Big Bang picture.

A popular T-shirt meme about atoms, a clever science pun!

A popular T-shirt meme about atoms, a clever science pun!

The second and third pillars of evidence for the Big Bang have to do with what happens to the Universe as it expands and cools, and the consequences for matter.  Everything you and I see around us here on Earth — rocks, trees, candy bars, platypuses — is made of atoms. Atoms are a composite structure. The center is a compact heavy core called the nucleus comprised of protons and neutrons. It is surrounded by a cloud of electrons, equal in number to the protons in the nucleus.

The fact that the atom holds together is a manifestation of the forces at work. The electrons are held to the atom by virtue of attractive electrical force between them and the nucleus. If you bang two atoms together hard enough, they break apart into free nuclei and free electrons.

The nuclei, built of protons and neutrons, are held together by a very strong force that acts over short distances called the nuclear force. The nuclear force is tremendously strong, but if you bang to nuclei together hard enough, they too can be broken up into free protons and free neutrons.

In the distant past, shortly after the Big Bang, the Universe was very compact: everything in it was closer together, and extraordinarily hot. As the Universe gets smaller, it’s a bit like being squished together in the mosh pit at a concert — you can’t really move anywhere without crashing into something else. The enormous temperatures mean that everything was moving extremely fast — the hotter the temperatures, the faster the motion, the harder the crashes. If you go far enough back in time, the Universe gets so hot you can’t have atoms. If you go even farther back, it gets hotter and the Universe can’t even have atomic nuclei.

The basic constituents made during Big Bang nucleosynthesis.

The basic constituents made during Big Bang nucleosynthesis. Protons (red) and neutrons (green) bind to form the simplest atomic nuclei.

The second pillar can be understood by going back to a time in the first minute after the Big Bang. Up to this point, the Cosmos was a primordial soup of free electrons, free neutrons, and free protons, all swirling around in a maelstrom of churning energy. The Universe had started its inexorable expansion, and was cooling as a result.  By the time the Cosmos was 10 seconds old, it had cooled from its hot beginnings down to a temperature of about 2 billion degrees Celsius. At this temperature, protons and neutrons begin to stick together to form the first atomic nuclei. This process of formation is called primordial nucleosynthesis — it makes hydrogen, deuterium, helium, and small amounts of lithium and beryllium.

Big Bang theory predicts how much of each of these was synthesized in the first 15-20 minutes, a delicate balance astronomers call the primordial abundances. What astronomers can see agrees with the predictions of Big Bang nucleosynthesis.

But perhaps the most important observational signature of the Big Bang has to do with light. After the formation of the atomic nuclei, the Cosmos was still too hot to form proper atoms. Every time a nucleus tried to bind with an electron, a collision would knock the electron free. So, for the next 400,000 years, the Universe remained a seething fluid of atomic nuclei, free electrons, and energy.

When light is packed in so tightly with charged particles, like the electrons and atomic nuclei, it is not free to travel about of its own free will. It travels only a short distance before it encounters an electron, and it scatters.  Light simply can’t go very far.

(L) Before recombination, light cannot travel very far because it encounters free electrons, which interact with it causing it to scatter. (R) After the electrons bind to nuclei to make atoms, the light decouples from matter, and is free to stream through the Universe unimpeded by scattering interactions.

(L) Before recombination, light cannot travel very far because it encounters free electrons, which interact with it causing it to scatter. (R) After the electrons bind to nuclei to make atoms, the light decouples from matter, and is free to stream through the Universe unimpeded by scattering interactions.

But, after 400,000 years, the Universe cools to a balmy 3000 degrees Celsius, cool enough that each time an electron bumps into a nucleus, it binds together to form an atom. This process is called recombination. From the point of view of the light, all of the charged particles suddenly disappear (atoms are neutral, having no overall electric charge) and the Universe becomes transparent. The light can travel anywhere it wants without being impeded by the matter; astronomers call this decoupling.  The Cosmos is full of freely streaming light.

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.

A map of the Cosmic Microwave Background across the whole sky. First detected by Penzias and Wilson in 1965, this light is the signature of ever cooling Universe after the Big Bang. The band of stronger microwave light along the center of the map is the signature of the Milky Way Galaxy.

For the next 13 billion years, the Universe continued to expand to the present day. All the while the streaming light — the signature from the birth of atoms — surfed right along.  Spacetime is stretching as the Cosmos expands, and the light from that early, hot, dense state had to give up energy to fight against the expansion, shifting to longer and longer wavelengths as time progressed. By the time light reaches the Earth today, it should appear as microwave light.  And indeed, it is. In every direction we look on the sky, we see a uniform background of microwave light called the Cosmic Microwave Background. This is the third, observational pillar, the evidence, that tells us our thinking about the Big Bang is on the right track.

This is the basic picture of the Big Bang that was developed since the late 1920’s — decades of careful comparison of observations with theoretical calculations. The refinements and developments — of both theory and experiment — continue to this day.

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This post is part of an ongoing series written for the General Relativity Centennial, celebrating 100 years of gravity (1915-2015).  You can find the first post in the series, with links to the successive posts in this series here: http://wp.me/p19G0g-ru.

Cosmos 10: The Edge of Forever

by Shane L. Larson

My parents are both natural scientists — my mother is a forester, and my father is a plant ecologist. As kids, we would spend long weeks in the summer camping doing “field research” in the wild backwoods of the Rockies.  Tents were optional, and at night I would often lie in my sleeping bag staring up into the blackest night you can imagine.  

The Milky Way, rising over a campsite near Crater Lake, Oregon. [Image by Shane Black, Website]

The Milky Way, rising over a campsite near Crater Lake, Oregon. [Image by Shane Black, Website]

From the dark valleys and meadows of the Rockies, the sky is a seemingly endless tapestry of black velvet, studded with a sparkling horde of stars converging on the vast gossamer seam of the Milky Way.  It is impossible to scan that deep darkness, as you slowly drift into that fuzzy netherworld between wakefulness and sleep, and not feel like you are falling into an infinitely deep sea that goes on forever.

And for all we know, it does go on forever! The vastness of the Cosmos, especially compared to the typical scales of our everyday lives, is mind-bogglingly large. It is a fact that we have always been cognizant of — our stories, our legends, even our everyday experiences place the sky very far away, beyond the simple reach of human hands. The glitter of distant stars provide an ideal tapestry upon which we can paint our wonderings about the Universe and our place within it. Where did the stars come from? Where did the Earth come from? Where did we come from?  These are the oldest questions we know of, uttered around campfires and over late night dinners and in scholarly classrooms for countless generations.  The answers to these questions are part of an exquisitely interlinked puzzle that starts with the birth of the Universe, and leads ultimately to me and you.  The study of that puzzle is called cosmology.

Cosmology is a branch of science that is a bit like history — we are reconstructing the past history of the Cosmos as a way to understand what we see around us today, and to predict what the ultimate future and fate of Everything might be.  We reconstruct that past history by looking deep into the Cosmos, and with the Laws of Nature in hand, attempt to explain what we see.  As we have talked about before, looking out into the Cosmos is a kind of Wayback Machine — looking across space is looking back in time.  Today, we can see farther across the Universe than at any other time in human history; we have discovered and know more than all the 40,000 generations of humans who have come before us.  And we’ve discovered something remarkable; we’ve discovered that in the beginning, something happened.  Cosmologists call that something, “The Big Bang,” the origin of Everything that Is.

bigBangWhen studying cosmology, you will often read a sentence about the Big Bang and the Origin of the Universe.  The Origin Statement goes something like this: everything in the Universe began in an infinitely dense point smaller than the period at the end of this sentence.  What does that mean?  The answer is the basis for our current understanding of all of the Cosmos.  Let’s parse that question into several smaller questions.

The first obvious question is what do we mean by Universe?  Here, we will take the fundamental definition of Universe to be “everything that exists.”  But there is an unspoken subtlety in the Origin Statement as we wrote it here — in this case, the use of the word “Universe” actually means “Observable Universe.”  What’s the difference between “The Universe” and “The Observable Universe?”

My favorite lunch dive, in Logan, UT.

My favorite lunch dive, in Logan, UT.

Consider an example from your everyday life — lunchtime.  Imagine one sunny day you decide to forego the sack lunch you brought with you and instead decide to go out to lunch with some of your friends.  You only have 1.5 hours before you have to be back, and you are walking on foot.  You can only walk so fast, so where are you going to go?  Perhaps Guy Fieri has pointed you toward an excellent BBQ joint on a late night episode of “Diners, Drive-Ins and Dives,” but that would require either a plane flight or a very long road trip, both of which will take far longer than the 1.5 hours you have. Instead, you confine your attention to restaurants within a certain distance — reachable if you walk as fast as you can for a limited amount of time.

The Universe is kind of the same way — there is a maximum speed that anything can attain, namely the speed of light. Thus, in the age of the Universe, there is a maximum distance over which any information can come to the shores of Earth — the distance that light can travel in the age of the Universe!  The “Observable Universe” is that part of the Universe from which we could have received some light that started travelling at the moment the Universe was born, and is just now reaching Earth today (analogous to how far you can walk during your lunch hour).  It is a small part of the “Entire Universe,” most of which we know nothing about because the light from there has not had the chance to reach us (analogous to the entire vast world full of restaurants, which you cannot reach during your lunch hour!).

The Observable Universe is just a small part of the Entire Universe. It is bounded by the farthest distance light could have travelled in the age of the Cosmos. If Earth is at the center of this boundary, then light from outside the blue boundary (such as from the yellow galaxy on the right) hasn't had time to reach us yet.

The Observable Universe is just a small part of the Entire Universe. It is bounded by the farthest distance light could have travelled in the age of the Cosmos. If Earth is at the center of this boundary, then light from outside the blue boundary (such as from the yellow galaxy on the right) hasn’t had time to reach us yet. [Illustration by S. Larson]

In the beginning, the Entire Universe was just a vast (possibly infinite) collection of the ultra-dense points mentioned in the Origin Statement.  The Big Bang was not really an explosion, not in the sense of an exploding stick of dynamite; the Big Bang was the apparently spontaneous and rapid expansion of every single point in the Entire Universe.  What is expanding?  The fabric that makes up the Universe itself is expanding, carrying everything that would become stars, galaxies, trees, kangaroos and people along with it.  Cosmologists call that fabric spacetime.  The very stuff that the Universe is made of — spacetime — is stretching.

Now if that doesn’t immediately make sense, don’t worry! It is a disconcerting and unfamiliar idea. The contemplation of big ideas is always a bit uncomfortable, because we’re stretching our brains in ways that it is not used to; that’s the way science works.  One way to help settle your mind around unfamiliar concepts and to build intuition is to appeal to analogies.  Analogies and metaphors are not perfect, but they help connect the ideas that need to be connected.  One of the classic analogies to understand the Big Bang is to imagine other things that stretch and expand.

Consider a large piece of spandex, with a checkerboard on it, as in the figure shown here.  The checkerboard pattern is not necessary, but it provides a quick and easy way for us to see and talk about distances. This checkered fabric is an analogy, a metaphor that we use to think about the Universe, and in this picture imagine it stretches far beyond the boundaries of the page of your computer screen.  For the moment, I have made the checkers large enough to see, but you could easily imagine them being smaller than what is drawn here, even much smaller (perhaps as small as the proverbial period at the end of the Origin Statement).

Imagine I have two ants sitting on the spandex, one named Xeno (the black ant) and one named Scarlett (the red ant).  They have both staked out a square they like, and are staying put, watching closely that the other ant does not move off their chosen territory.  This is the case shown in the first figure.

(L) Consider two ants, Xeno and Scarlett, on a stretchy sheet representeding the spacetime fabric of the Cosmos. (R) When the Cosmos expands in every direction and at every point, the two ants get farther apart. [Illustration by S. Larson]

(L) Consider two ants, Xeno and Scarlett, on a stretchy sheet representeding the spacetime fabric of the Cosmos. (R) When the Cosmos expands in every direction and at every point, the two ants get farther apart. [Illustration by S. Larson]

Now, unbeknownst to our ants, the very fabric of the Universe is expanding around them, as shown in the second figure. It expands uniformly, in every direction. The result of that, in the context of my spandex checkers, is that every square gets larger (though our ants remain the same size, comfortably bound together by the biological goop and intermolecular forces that give their bodies form).  What are the observational consequence for our ants, keeping their beady little ant eyes on each other?  Much to their surprise, they find themselves slowly getting farther apart!  Scarlett looks around, and clearly she is not moving — she has not moved at all since our little experiment began. Never-the-less, it is quite clear that Xeno is receding from her. Meanwhile, Xeno is thinking the same thing. He has not shifted nor moved at all, but Scarlett is inexorably getting farther away.  The explanation? The very space between them, the stuff that the Universe is made of, is expanding.

What is interesting is that every square in the fabric of our Universe is expanding — the squares are getting larger, and everything is getting farther apart.  No matter how tiny every square started, if we wait long enough, it gets bigger.  The consequence is that from the perspective of anyone anywhere in the Universe, every other point is flying away from them.  Consider a few more ants: Xeno, Scarlett, Kermit (the green ant) and Indigo (the blue ant). If each one of them measures the distance to every other ant, they find that if they wait a while, the distance to every other ant increases.  From  the perspective of any ant, firmly rooted to their little territory in the Cosmos, every other point in the Universe is slowly getting farther and farther away, no matter what direction they look.

Imagine an army of ants (clockwise from the top: Kermit, Scarlett, Indigo, and Xeno). If they all watch each other as the Universe expands, they think ALL other ants are moving away from them, no matter what direction they are.

Imagine an army of ants (clockwise from the top: Kermit, Scarlett, Indigo, and Xeno). If they all watch each other as the Universe expands, they think ALL other ants are moving away from them, no matter what direction they are. [Illustration by S. Larson]

This is how we think about the Big Bang.  Everything that you can see (the Observable Universe) was once contained in a dot smaller than the period at the end of this sentence; it was like a teeny, tiny square on our spandex.  Then the Big Bang happened, and every point in the Entire Universe — every point on the spandex — started to expand.  The part of the Universe you can see is only one small part of the vast fabric that is everything, but it all started long ago in a very tiny spot.  Everything you can see in the Universe began in an infinitely dense point smaller than the period at the end of this sentence!

On the surface, this story sounds fantastical, almost beyond belief. We can always make up fantastical ideas about the nature of the Cosmos, but for those ideas to move beyond mere speculation and into the realm of science, we must be able to test those ideas.  There must be something we can look for, something that we can observe. In the case of cosmology, there is.

crunchOne of the things that physicists know about the world is that if you compress things they get hot. This is the principle behind pressure cookers, this is why it is hot in the core of the Earth, and this is why the Sun burns hydrogen in its core. When the pressure goes up, things get hot!  If the Universe is expanding today, we can imagine running the movie backward in time, watching everything run backward toward the Big Bang.  Because we see everything flying apart now, when we run the movie backward what we see is the entire Observable Universe being compressed down into a small point. The pressure in that point would have been enormous, which means it would have been tremendously hot.  If that were true, there should be some thermal signature of that early, hot, dense state of the Cosmos.

There is such a signature. Arriving on Earth from every direction on the sky, is a faint fog of microwave radiation, known as the Cosmic Microwave Background. It is the light that was released from the birth of all the atoms in the Cosmos, 400,000 years after the Big Bang.  Before this time, the Universe was so hot and dense that atoms could not hold together; they would constantly crash together and break apart into the fluff from which they are made, melting back into the primordial soup of light and sub-atomic particles. But as the Universe expands, it cools slowly until atoms could hold together. The moment that happened, the soup immediately thinned and the light flew free, carrying the message of the birth of the atoms.

The Planck map of the Cosmic Microwave Background.

The Planck map of the Cosmic Microwave Background. [ESA/Planck Collaboration]

This picture of the Cosmic Microwave Background is the most accurate map every made of the microwave sky; it is the youngest picture of the Cosmos we have ever taken, and the strongest piece of evidence we have that the Big Bang unfolded in the way we have just discussed. This picture, an image of the Cosmos very shortly after its birth, is one of the greatest legacies of our race. It captures, in an exquisite map of subtle patterns and colors, the ability of our species to reduce our ignorance, to become more enlightened about the Cosmos and our place in it.

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This post is part of an ongoing series, celebrating the forthcoming science series, Cosmos: A Spacetime Odyssey by revisiting the themes of Carl Sagan’s classic series, Cosmos: A Personal Voyage.  The introductory post of the series, with links to all other posts may be found here:  http://wp.me/p19G0g-dE