What Was It Like When Dark Energy First Took Over The Universe?
“In reality, we can only make observations at one point in time: today, or when the light from all the distant objects throughout the Universe is finally reaching us. But we can imagine our hypothetical scenario just as well.
What would we see if we could track a single, individual galaxy — including both its distance and its redshift as seen from our perspective — throughout the history of the Universe?
The answer may be a little counterintuitive, but it’s tremendously illustrative and educational as far as shedding light on not only what dark energy is, but how it affects the expansion of the Universe.”
If you could put your finger down on a distant, individual galaxy far away and unbound from our own, what would you see if you could track its motion over time? For the first few billion years, it would be moving away very quickly, getting more and more distant, but it would appear to slow down. It would be as though gravity were trying to pull it back to us, albeit unsuccessfully. And then, at a critical moment some 7.8 billion years after the Big Bang, this slowing down would cease. The galaxy would transition from decelerating to accelerating, and would speed away from us, faster and faster, ever after.
This marks the transition to when dark energy took over the expanding Universe. Come find out what it was like when that happened, and how, today!
What Was It Like When The Universe Made Its Heaviest Elements?
“For a long time, it was speculated that merging neutron stars would provide the origin of these elements, as two massive balls of neutrons smashing together could create an endless variety of heavy atomic nuclei. Sure, most of the mass from these objects would merge together into a final-stage object like a black hole, but a few percent should be ejected as part of the collision.
In 2017, observations made with both telescopes and with gravitational wave observatories confirmed that not only are neutron star mergers responsible for the overwhelming majority of these heavy elements, but that short-period gamma ray bursts can be linked to these mergers as well. Now known as a kilonova, it’s well-understood that neutron star-neutron star mergers are the origin of the majority of the heaviest elements found throughout the Universe.”
For those of you keeping track, this is the 22nd article I’ve written in my “what was it like when…” series. There’s an entire past and future history of our Universe to tell, and we haven’t even reached the present day.
Enjoy the story of how we made the heaviest elements of all, and stay tuned for even more.
What Was It Like When The Milky Way Took Shape?
“The cosmic story that led to the Milky Way is one of constant evolution. We likely formed from hundreds or even thousands of smaller, early-stage galaxies that merged together. The spiral arms likely formed and were destroyed many times by interactions, only to re-form from the rotating, gas-rich nature of an evolving galaxy. Star formation occurred inside in waves, often triggered by minor mergers or gravitational interactions. And these waves of star-formation brought along increases in supernova rates and heavy metal enrichment. (Which sounds like everyone’s favorite after-school activity.)
These continuous changes are still occurring, and will come to a conclusion billions of years in the future, when all the galaxies of the Local Group have merged together. Every single galaxy has its own unique cosmic story, and the Milky Way is just one typical example. As grown up as we are, we’re still evolving.”
We normally think of events in the past of having occurred at a specific time. Star formation began in the Universe when it was 50-to-100 million years old. The first galaxies formed some ~200 million years after that. The Universe became transparent to visible light 550 million years after the Big Bang, and star formation reached its maximum between 2 and 3 billion years after the Big Bang.
But when did the Milky Way form?
That’s a silly question, as it turns out, because what we know as the Milky Way has been constantly evolving and growing over time. Had we come along billions of years ago, or were we to come along billions of years in the future, our galaxy would be unrecognizable to us.
Here’s the story of how the Milky Way took shape, and what it was like along every step of the way. You might be surprised!
What Was It Like When The Cosmic Web Took Shape?
“Although the seeds necessary for cosmic structure were planted in the very earliest stages of the Universe, it takes time and the right resources for those seeds to grow to fruition. The seeds for small-scale structure germinate first, as the gravitational force propagates at the speed of light, growing overdense regions into the earliest star clusters after only a few tens of millions of years. As time goes on, the seeds for galaxy-scale structure grow too, taking hundreds of millions of years to bring about galaxies within the Universe.
But galaxy clusters, growing from the same magnitude seeds on larger distance scales, take billions of years. By time the Universe is 7.8 billion years old, the accelerated expansion has taken over, explaining why there are no larger bound structures than galaxy clusters. The cosmic web is no longer growing as it once was, but is primarily being torn apart by dark energy. Enjoy what we have while we have it; the Universe will never be this structured again!”
When we look out at the Universe today, we find stars bound together in enormous collections known as galaxies, and galaxies clumped together into groups and clusters, which themselves appear to be connected by filaments of matter. This cosmic web took many billions of years to form, though, and the smaller-scale structure formed far earlier in the Universe than the large-scale structure. While it took only tens of millions of years for stars and hundreds of millions for galaxies, it took billions of years for galaxy clusters, and anything you’ve heard about ‘superclusters’ is a mere phantasm.
Come learn what it was like when the cosmic web took shape, and how the Universe came to appear the way it is today!
What Was It Like When The First Habitable Planets Formed?
“The galactic center, however, is a relatively difficult place for a planet to be considered habitable beyond a reasonable doubt. Wherever you have stars continuously forming, you have a spectacular slew of cosmic fireworks. Gamma ray bursts, supernovae, black hole formation, quasars, and collapsing molecular clouds make for an environment that is, at best, precarious for life to arise and sustain in.
To have an environment where we can confidently state that life arises and maintains itself, we need for this process to come to an abrupt end. We need something to put a stop to star formation, which in turn puts the kibosh on the activity that is most threatening to habitability on a world. It’s why the earliest, most sustained habitable planets might not be in a galaxy like ours, but rather in a red-and-dead galaxy that ceased forming stars billions of years ago.”
The cosmic story that created the Universe as we know it had a lot of intricate and fascinating steps along the way. The stars needed to live and die to create heavy elements; enough elements needed to form to make life and rocky planets possible; and the Universe needed to quiet down enough in the richest, locations so that life could sustain and thrive. That last step takes surprisingly long relative to the first few! While rocky planets might come into being less than half-a-billion years after the Big Bang, and life might be able to arise in under a billion years, having the right combination of planets that are habitable and continuously hospitable to life might take up to two billion years, even in the most optimistic of circumstances.
Still, that’s 7 billion years faster than it took for Earth to form! What could life in the Universe that got such a head start on us look like? Consider the possibilities as you learn what it was like when the first habitable planets formed!
What Was It Like When Galaxies Formed The Greatest Number Of Stars?
“The star-formation rate declined slowly and steadily for a few billion years, corresponding to an epoch where the Universe was still matter-dominated, just consisting of more processed and aged material. There were fewer mergers by number, but this was partially compensated for by the fact that larger structures were merging, leading to larger regions where stars formed.
But right around 6-to-8 billion years of age, the effects of dark energy began to make their presence known on the star formation rate, causing it to plummet precipitously. If we want to see the largest bursts of star formation, we have no choice but to look far away. The ultra-distant Universe is where star formation was at its maximum, not locally.”
In a myriad of locations, throughout our galaxy and almost all the galaxies in the known Universe, new stars form wherever a cloud of gas is triggered into collapsing. From the Orion Nebula to dozens of others in our own galaxy, new stars form thousands-at-a-time in regions all throughout our local neighborhood. But as spectacular as these sights are, they’re much, much rarer than they were a long time ago. In fact, we formed stars at a rate that was 30 times faster than today back when the Universe was young. For the last 11 billion years, we’ve been forming fewer and fewer stars everywhere we look.
The Universe is changing even today, and fewer and fewer stars are being newly created as time goes on. There are many reasons why; come learn them today!
What Was It Like When The First Supermassive Black Holes Formed?
“The earliest galaxies and quasars we’ve ever found are among the brightest, most massive ones we expect to exist. They are the great winners in the gravitational wars of the early Universe: the ultimate cosmic overdogs. By time our telescopes reveal them, 400-to-700 million years after the Big Bang (the earliest quasar comes from 690 million years), they already have billions of stars and supermassive black holes of many hundreds of millions of solar masses.
But this is not a cosmic catastrophe; this is a piece of evidence that showcases the runaway power of gravitation in our Universe. Seeded by the first generation of stars and the relatively large black holes they produce, these objects merge and grow within a cluster, and then grow even larger as clusters merge to form galaxies and galaxies merge to form larger galaxies. By today, we have black holes tens of billions as massive as the Sun. But even in the earliest stages we can observe, billion-solar-mass black holes are well within reach. As we peel back the cosmic veil, we hope to learn exactly how they grow up.”
One of the great challenges for modern astrophysics and cosmology is to explain where these behemoths at the centers of galaxies, the supermassive black holes found throughout the Universe, came from. How did they get so big? And how did they get so big so fast? It might seem reasonable to get a few billion solar mass black holes by today, 13.8 billion years after the Big Bang. But how could you get something nearly that large when the Universe was just 5% of its current age? Through the physics of the first stars, first galaxies, and gravitational attraction and collapse, of course!
Come get the cosmic story of where these ultimate objects come from, and learn how it really is possible without any new physics!
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.
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?)