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 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 Inflating?
“In theory, what lies beyond the observable Universe will forever remain unobservable to us, but there are very likely large regions of space that are still inflating even today. Once your Universe begins inflating, it’s very difficult to get it to stop everywhere. For every location where it comes to an end, there’s a new, equal-or-larger-sized location getting created as the inflating regions continue to grow. Even though most regions will see inflation end after just a tiny fraction of a second, there’s enough new space getting created that inflation should be eternal to the future.”
You’ve no doubt heard that the overwhelming scientific consensus is that the observable Universe began with the hot Big Bang. What’s far less common, but just as overwhelmingly accepted and well-understood, is that a period of cosmological inflation occurred prior to the Big Bang in order to set it up. While most of us can visualize the expanding Universe fairly well, it’s much more difficult to get a good handle on what the Universe looked like during the epoch of cosmic inflation. Yet if you want to know where our Universe came from, and how it was born with the properties our hot Big Bang started off with, that’s exactly the challenge you have to meet.
Here’s an in-depth but scientifically accurate description of what the Universe was like when inflation occurred, and how it gives us the Universe we inhabit today!