This Is How We Know The Cosmic Microwave Background Comes From The Big Bang
“The outer layers are extremely tenuous and rarified, and the radiation we receive here on Earth doesn’t all originate from the very edge of that plasma. Instead, much of what we see originates from about the first 500 kilometers, where the interior layers are significantly hotter than the outermost ones. The light coming from our Sun — or any star, for that matter — is not a blackbody, but the sum of many blackbodies that vary in temperature by many hundreds of degrees.
It’s only when you add all these blackbodies together that you can reproduce the light we see coming from our parent star. The cosmic microwave background, when we look at its spectrum in detail, is a far more perfect blackbody than any star could ever hope to be.”
If you get your science from the internet, you might hear about all sorts of alternatives to the Big Bang. Grandiose claims are often made, decrying the Big Bang as a religion that can never be falsified, while simultaneously touting ideas that most scientists discarded decades or even centuries ago.
But there is no ideology at play; science is a game that we play with predictive power and evidence. The Big Bang makes explicit predictions, and so do alternative ideas that rely on atomic emissions, reflected starlight, photonic energy loss, or heated-up dust.
We can look at every idea we can conceive of, but in the end, only one matches what we observe. Here’s how the Cosmic Microwave Background points to the Big Bang, and away from every other alternative.
What Was It Like When The First Stars Began Illuminating The Universe?
“After the Big Bang, the Universe was dark for millions upon millions of years; after the glow of the Big Bang fades away, there’s nothing that human eyes could see. But when the first wave of star formation happens, growing in a cosmic crescendo across the visible Universe, starlight struggles to get out. The fog of neutral atoms permeating all of space absorbs most of it, but gets ionized in the process. Some of this reionized matter will become neutral again, emitting light when it does, including the 21-cm line over timescales of ~10 million years.
But it takes far more than the very first stars to truly turn on the lights in the Universe. For that, we need more than just the first stars; we need them to live, burn through their fuel, die, and give rise to so much more. The first stars aren’t the end; they’re the beginning of the cosmic story that gives rise to us.”
We like to think of the Universe evolving as a story that follows a particular order: first we had the Big Bang, then things expanded and cooled, then gravitation pulled things into clumps, we formed stars, they lived and died, and now here we are. But in reality, things are messier than that! The very first stars didn’t immediately spread light throughout the Universe, but instead had a cosmic ocean of neutral atoms to contend with: one that they weren’t energetic enough or numerous enough to break through. The first stars in the Universe fought a battle against the clumping, neutral, atomic-based matter that surrounded them… and lost.
Come get the valiant but ultimately unsuccessful story of the first stars in the Universe, and learn why “letting there be light” didn’t illuminate the Universe!
This Is Why There Are No Alternatives To The Big Bang
“For more than 50 years, no alternative has been able to deliver on all four counts. No alternative can even deliver the Cosmic Microwave Background as we see it today. It isn’t for lack of trying or a lack of good ideas; it’s because this is what the data indicates. Scientists don’t believe in the Big Bang; they conclude it based on the full suite of observations. The last adherents to the ancient, discredited alternatives are at last dying away. The Big Bang is no longer a revolutionary endpoint of the scientific enterprise; it’s the solid foundation we build upon. It’s predictive successes have been overwhelming, and no alternative has yet stepped up to the challenge of matching its scientific accuracy in describing the Universe.”
The last adherents to alternative theories to the Big Bang are at last dying away. Advocates of tired light, steady-state, or plasma cosmologies have ceased arising among the scientific ranks for one reason: these ideas cannot even explain the Cosmic Microwave Background observations, much less the full suite of the four major cornerstones of the Big Bang. When all we had were Hubble’s data and the evidence for the expanding Universe, it was a great idea to explore all the conceivable alternatives. Now that the data has come in, the alternatives have been scientifically falsified, and the Big Bang is the foundation we use as the base for our future theorizing.
This may disappoint some, but for the scientifically-minded among us, it’s a monument to the success of a fantastic theory. Here’s the scientific story of why no alternatives remain.
What Was It Like When The Universe Made Its First Elements?
“The Universe does form elements immediately after the Big Bang, but almost all of what it forms is either hydrogen or helium. There’s a tiny, tiny amount of lithium left over from the Big Bang, since beryllium-7 decays into lithium, but it’s less than 1-part-in-a-billion by mass. When the Universe cools down enough that electrons can bind to these nuclei, we’ll have our first elements: the ingredients that the very first generations of stars will be made out of.
But they won’t be made out of the elements we think of as essential to existence, including carbon, nitrogen, oxygen, silicon and more. Instead, it’s just hydrogen and helium, to the 99.9999999% level. It took less than four minutes to go from the start of the hot Big Bang to the first stable atomic nuclei, all amidst a bath of hot, dense, expanding-and-cooling radiation. The cosmic story that would lead to us has, in truth, finally begun.”
The first stars wouldn’t form until somewhere between 50 and 100 million years after the Big Bang, but the elements that made them up were created in just the first 3-to-4 minutes. When the Universe was a fraction of a second old, there was a 50/50 split between protons and neutrons; when it was 3 seconds old, it was more like 85/15. But all of those protons and neutrons couldn’t just fuse together to form deuterium, helium, and then the heavier elements like they do in stars, even though the Universe was energetic and dense enough to make that happen.
Instead, we wound up with just hydrogen, helium, and less than 0.0000001% anything else. This is the story of how.
What Was It Like When We Lost The Last Of Our Antimatter?
“The Cosmic Microwave Background’s temperature was first measured to this precision back in 1992, with the first data release of NASA’s COBE satellite. But the neutrino background imprints itself in a very subtle way, and wasn’t detected until 2015. When it was finally discovered, the scientists who did the work found a phase shift in the Cosmic Microwave Background’s fluctuations that enabled them to determine, if neutrinos were massless today, how much energy they’d have at this early time.
Their results? The Cosmic Neutrino Background had an equivalent temperature of 1.96 ± 0.02 K, in perfect agreement with the Big Bang’s predictions.”
Throughout the very early Universe, space was filled with matter and antimatter, which spontaneously self-create from pure Energy via Einstein’s famous E = mc^2. However, as the Universe cools and expands, less energy becomes available to make new particles and antiparticles. Quarks, muons, taus, baryons, mesons, and gauge bosons all are gone by time the Universe is just 25 microseconds old. But positrons, the counterpart of antielectrons, remain until the Universe is a full 3 seconds old! Their existence leads to a crazy prediction: that there should be a cosmic neutrino background at a different temperature from the cosmic microwave background: 1.95 K instead of 2.73 K.
We have verified this, and hence, one of the Big Bang’s craziest predictions, with data collected 13.8 billion years onward! Come learn what it was like when the Universe lost the last of its antimatter.
First Stars Formed No Later Than 250 Million Years After The Big Bang, With Direct Proof
“We see MACS1149-JD1 as it was 530 million years after the Big Bang, while inside, it has a special signature: oxygen. Oxygen is only produced by previous generations of stars, indicating that this galaxy is already old.
MACS1149-JD1 was imaged with microwave (ALMA), infrared (Spitzer), and optical (Hubble) data combined.
The results indicate that stars existed nearly 300 million years before our observations.”
One of the great quests of astronomers today is to measure and locate the very first stars in the Universe. As far back as Hubble can see, to when the Universe was just 3-5% its current age, the Universe is still full of galaxies, even though they’re smaller and bluer than the ones we have today. But within these galaxies, we can also find evidence that the stars in there aren’t the very first ones; they contain evidence for prior generations of stars in their spectral signatures. From the second-most distant galaxy ever discovered, itself just 530 million years after the Big Bang, we see evolved stars. They indicate that the very first ones formed no later than 250 million years after the Big Bang.
The James Webb Space Telescope will be able to see that far! In less than 3 years, we’ll peer beyond where we’ve ever seen before. And there will no doubt be something breathtaking to look at.
What Was It Like When The Universe First Created More Matter Than Antimatter?
“This is only one of three known, viable scenarios that could lead to the matter-rich Universe we inhabit today, with the other two involving new neutrino physics or new physics at the electroweak scale, respectively. Yet in all cases, it’s the out-of-equilibrium nature of the early Universe, which creates everything allowable at high energies and then cools to an unstable state, which enables the creation of more matter than antimatter. We can start with a completely symmetric Universe in an extremely hot state, and just by cooling and expanding, wind up with one that becomes matter-dominated. The Universe didn’t need to be born with an excess of matter over antimatter; the Big Bang can spontaneously make one from nothing. The only open question, exactly, is how.”
One of the biggest unsolved questions in physics today is how the Universe came to be filled with matter and not antimatter. After all, the laws of physics are completely matter-antimatter symmetric, and yet when we look at what we have today, every planet, star, and galaxy is made of matter and not antimatter. How did it come to be this way? The young, hot, but rapidly expanding-and-cooling Universe gives us all the ingredients we need for this to occur. We are certain of the exact mechanism, but theoretically, there are some enticing possibilities. Here’s a walk through one of those scenarios in great detail, but expressed so simply that even someone with no physics knowledge can follow it.
Here’s what the Universe was like when it was matter-antimatter symmetric, along with how it could have become matter-rich without breaking the laws of physics.
Ask Ethan: How Large Is The Entire, Unobservable Universe?
“We know the size of the Observable Universe since we know the age of the Universe (at least since the phase change) and we know that light radiates. […] My question is, I guess, why doesn’t the math involved in making the CMB and other predictions, in effect, tell us the size of the Universe? We know how hot it was and how cool it is now. Does scale not affect these calculations?”
Our Universe today, to the best of our knowledge, has endured for 13.8 billion years since the Big Bang. But we can see farther than 13.8 billion light years, all because the Universe is expanding. Based the matter and energy present within it, we can determine that the observable Universe is 46.1 billion light years in radius from our perspective, a phenomenal accomplishment of modern science. But what about the unobservable part? What about the parts of the Universe that go beyond where we can see? Can we say anything sensible about how large that is?
We can, but only if we make certain assumptions. Come find out what we know (and think) past the limits of what we can see on this week’s Ask Ethan!
What Was It Like When The Universe Was At Its Hottest?
“At the inception of the hot Big Bang, the Universe reaches its hottest, densest state, and is filled with matter, antimatter, and radiation. The imperfections in the Universe — nearly perfectly uniform but with inhomogeneities of 1-part-in-30,000 — tell us how hot it could have gotten, and also provide the seeds from which the large-scale structure of the Universe will grow. Immediately, the Universe begins expanding and cooling, becoming less hot and less dense, and making it more difficult to create anything requiring a large or energy: E = mc2 means that creating a massive particle requires at least enough energy.
Over time, the expanding and cooling Universe will drive an enormous number of changes. But for one brief moment, everything was symmetric, and as energetic as possible. Somehow, over time, these initial conditions created the entire Universe.”
As soon as the Universe was filled with matter, antimatter, and radiation in the hot, dense state known as the Big Bang, it begins to expand and cool. For one brief moment, the Universe reached its maximum temperature and density, and had enough energy to spontaneously create anything at all that Einstein’s energy-mass equivalence would allow. But this state not only wouldn’t last, but it also was never arbitrarily or infinitely hot! There’s a limit to how energetic the Universe could have ever been, and we’ve determined it’s at least 1000 times smaller than the Planck scale. This is still trillions of times more energetic than anything the LHC ever created.
What was it like when the Universe was the hottest its ever been? Come find out on What-Was-It-Like-Whensday! (See what I did there?)
What Was It Like When The Big Bang First Began?
“Once inflation comes to an end, and all the energy that was inherent to space itself gets converted into particles, antiparticles, photons, etc., all the Universe can do is expand and cool. Everything smashes into one another, sometimes creating new particle/antiparticle pairs, sometimes annihilating pairs back into photons or other particles, but always dropping in energy as the Universe expands.
The Universe never reaches infinitely high temperatures or densities, but still attains energies that are perhaps a trillion times greater than anything the LHC can ever produce. The tiny seed overdensities and underdensities will eventually grow into the cosmic web of stars and galaxies that exist today. 13.8 billion years ago, the Universe as-we-know-it had its beginning. The rest is our cosmic history.”
The Big Bang is normally treated as the very beginning of the Universe, but in reality there’s a phase that came before the hot Big Bang to set it up. During cosmic inflation, the Universe was filled with an extremely large amount of energy inherent to space itself, causing the Universe to inflate, stretch flat, and achieve almost exactly the same properties everywhere. The Universe we have today, however, is full of matter and radiation, and originated in a hot Big Bang 13.8 billion years ago. How did we go from this inflating state to our hot, dense, uniform and expanding-and-cooling Universe?
This tells you the best scientific story of how we got there, along with an in-depth description of what it was like at those first moments where our Universe gave us something to look at.