This Is Why December 31 Is The Cosmic Calendar's Most Important Date
By Ethan Siegel - 31.
The Universe is out there, waiting for you to discover it.
Our Universe has been around for 13.8 billion years since the Big Bang. This timespan is so incredibly long and so far outside our normal human experience that most of us can't even wrap our heads around it.
Speaking about events that happened thousands, millions, or billions of years ago might all seem unfathomably ancient, but they're as different from one another as they are from what happened this past year.
However, we can leverage a fun thought-experiment to help us wrap our heads around the history of the Universe: imagine that all of it — all 13.8 billion years — were compressed to fit into a single calendar year.
Each "day" on that calendar would last around 38 million years, and a single human lifetime would last just about 0.2 seconds, on average. If this were how things truly unfolded, December 31st would be the most important date of all.
The history of our Universe would begin with the start of the hot Big Bang on January 1st, followed by the Universe expanding, cooling, and gravitating ever since.
The first stars would form on January 3rd, with the first galaxies forming a few cosmic days after. By the end of January, the first galaxy clusters are starting to form, followed by star formation reaching its peak in mid-March.
Our entire cosmic history is theoretically well-understood in terms of the frameworks and rules that govern it. It's only by observationally confirming and revealing various stages in our Universe's past that must have occurred, like when the first elements formed, when atoms became neutral, when the first stars and galaxies formed, and how the Universe expanded over time, that we can truly come to understand what makes up our Universe and how it expands and gravitates in a quantitative fashion. The relic signatures imprinted on our Universe from an inflationary state before the hot Big Bang give us a unique way to test our cosmic history, subject to the same fundamental limitations that all frameworks possess. Nicole Rager Fuller / National Science Foundation
At the end of June, the matter density drops enough that dark energy's effects begin to show and the Universe begins accelerating, sealing our cosmic fate. In early September, the Sun and Earth form, with the Moon forming from a giant impact about 1 cosmic day later. And yet, Earth's rich biological history only gets familiar towards the very end.
Mitochondria, which are some of the specialized organelles found inside eukaryotic cells, are themselves reminiscent of prokaryotic organisms. They even have their own DNA (in black dots), cluster together at discrete focus points. With many independent components, a eukaryotic cell can thrive under a variety of conditions that their simpler, prokaryotic counterparts cannot. But there are drawbacks to increased complexity, too. Francisco J Iborra, Hiroshi Kimura and Peter R Cook (Biomed Central LTD)
The first sexually reproducing organism, a single-celled eukaryote, doesn't evolve until December 2. The Cambrian explosion occurs on December 17. A giant impact occurs, rendering the non-avian dinosaurs extinct, on the morning of December 30th. But December 31st, which wouldn't even see the first Homo sapiens appear until 11:53 PM, is truly the most important day as far as our story is concerned.
As the clock passes midnight on December 30th, the world has largely moved on from yesterday's mass extinction event. Primates, rodents, lagomorphs, and many other forms of mammals — placental mammals, marsupials, and even egg-laying mammals — have taken that opportunity to diversify and fill every niche available. Wet-nosed and dry-nosed primates have already split, and the dry-nosed primates, right at (or perhaps just before) the start of December 31, split into New World and Old World monkeys.
The golden-headed lion tamarin is an example of a New World monkey. This endangered species of animal is part of a lineage that split off from the Old World monkeys that humans are a part of some 40 million years ago, or right around the start of December 31 on our cosmic calendar. Getty images
The Old World monkeys continue to occupy a variety of niches, living mostly in the trees and diversifying in terms of both size and physical appearance. At about 8 AM on December 31, the first apes arise, splitting off from the remaining Old World monkeys at this time. The apes — defined by the complete lack of a tail of any type — would go on to give rise to many of the close relatives of humans that survive today: both the lesser apes and the great apes.
Right around noon, the earliest ape to split off from the Old World monkeys — the Gibbon — arises. About 2 hours later, the first great apes arose, with the Orang-utan branching off at around 3 PM and spreading into southern Asia. The other great apes remained in Africa, with the largest primate of all-time, Gigantopithecus, arising at about 6 PM.
The Orang-utans are some of the earliest great apes to split off from our hominid ancestors, which they did some 16 million years ago. Although they are true great apes like we are, having no tails, they are less closely related to us than gorillas, bonobos, chimpanzees, or any of the later-emerging members of the family hominidae. Getty Images
At around 7:30 PM, the gorilla branched off from the other great apes, while a tremendously important evolutionary split occurred at about 8 PM: the split between the chimpanzee/bonobo branch and the branch of the great apes that would give rise to humans. The chimpanzee/bonobo branch remained unified until about 10:30 PM, with the extant chimpanzees and bonobos surviving as humanity's closest living relatives.
That's because all of the other, more direct ancestors of human beings, despite taking a rich evolutionary path before they arrived at us, no longer survive. In our evolutionary ascent, every member of the same family, genus, and species of human beings has been a casualty of both nature and our own activity. While chimpanzees and bonobos remain, we have no living relative that we share an evolutionary history with prior to 8 PM on December 31.
Bonobos, along with chimpanzees, are the two species most closely related to human beings that remain on Earth today. Bonobos are incredibly social, but still are not truly bipedal, as they maneuver on four limbs frequently. An evolutionary split that occurred approximately 5.6 million years ago marks the divergence of these creatures from modern humans. Getty Images
Just those last 4 hours before midnight on the cosmic calendar are filled with enormous and profound developments. At about 8:30 PM, the first truly bipedal ape, Ardipithecus, arose. An hour later, at 9:30 PM, the first Australipithecus evolved, marking the first appearance of the Hominina subtribe. At around 9:45 PM, we see the first evidence for stone tool use in human ancestors, a development that (in real-time) dates back to around 3.6 million years ago.
And then, at around 10:15-10:30 PM, and enormously critical step in our development occurred. Our hominid ancestors, facing food shortages, underwent two evolutionary paths: one branch developed stronger jaws, enabling them to crack previously uncrackable nuts, while the other developed larger brains and weaker jaws, providing a different path for food access. That latter path eventually led to the genus Homo, exemplified by the large-brained Homo habilis, while the strong-jawed branch quickly died out.
The group of hominids shown here includes many of our direct ancestors and evolutionary cousins. Shown here are Homo sapiens (modern humans), Australopithecus afarensis (thought to be the direct ancestor of the genus Homo), Homo erectus (which arose 1.9 million years ago and only died out ~140,000 years ago), Homo habilis (the first member of the genus Homo), and the Neanderthal (which arose later than, and independently of, modern humans). Encyclopaedia Britannica / UIG via Getty Images
By about 10:45 PM, Homo erectus had evolved, the largest-brained human ancestor so far and the first to both leave the African continent and display evidence of fire use. By 11:20 PM, Homo habilis and all of the Australopithecus species had become extinct.
But development still continued unabated, both evolutionarily and from a cultural/scientific perspective. At around 11:33 PM, the earliest evidence for cooking appears. At 11:36 PM, Homo heidelbergensis has evolved, thought by many to be an in-between link between modern humans and the older Homo erectus. And at 11:40 PM, the first evidence for clothing appears in the fossil record. It wasn't until approximately 11:48 PM on the cosmic calendar, or about 300,000 years ago in actual time, that anatomically modern humans, Homo sapiens, arose for the first time.
The oldest Homo sapiens fossils now date back to 300,000-315,000 years ago, and were found in Morocco. This find, dating back to only 2017, pushes back our species' origin earlier than the development of the neanderthals, and suggest that we didn't evolve only in East Africa, as previously believed. NHM London / Nature
What we consider the crowning achievements of civilization all only happened in the final minutes of the cosmic calendar.
- At 11:56 PM, the most recent glacial period arrived, forcing all surviving hominid populations towards equatorial latitudes.
- At 11:58:20 PM, 100 seconds before midnight on the cosmic calendar, modern humans left Africa for Europe for the first time.
- Between 11:58:30 and 11:58:54 PM, just before the final minute, we find the earliest musical instruments (a bone flute), domesticated dogs, cave paintings, and sculptures.
Interspersed in that penultimate minute, at 11:58:43 PM, the last Neanderthal died out, leaving Chimpanzees and Bonobos as humanity's closest living relative. Only in this last minute of cosmic history, corresponding to the most recent 26,000 years, did what we now consider "modern human civilization" emerge.
A picture taken on March 26, 2018 shows tools displayed for the Neanderthal exhibition at the Musee de l'Homme in Paris. Neanderthals and humans coexisted for thousands of years in Europe, but the extinction of the Neanderthals was swift and final in the aftermath of their encounters with human beings. STEPHANE DE SAKUTIN/AFP/Getty Images
This last minute of cosmic time sees the last glacial period ending, transforming the Earth to its modern global appearance. Humans spread to the Americas and Australia, while more large animals (like sheep, pigs, and goats) become domesticated as others (like the woolly mammoth) go extinct.
- 26 seconds before midnight, the last of the continental ice retreats, officially ending the last ice age.
- 22 seconds before midnight, farming and agriculture become widespread.
- 20 seconds before midnight, the first walled cities arise, with populations exceeding 1,000 humans.
- 18 seconds before midnight, pottery and winemaking show up.
- 14 seconds before midnight, the plough is invented, revolutionizing agriculture.
- 12 seconds before midnight, the wheel, numbers, and writing are invented.
- 9 seconds to midnight, metal working and the bronze age arrive.
On the cosmic calendar, each second corresponds to 440 years. Even landing on the Moon occurred barely 0.1 seconds ago, from a cosmic perspective.
This cosmic timeline, assembled by E. Siegel in 2014, is slightly out-of-date in some regards but still provides an outstanding visualization of how insignificant humanity's contributions are on a cosmic scale. E. Siegel
All that human beings have ever accomplished occurs in a cosmic blink-of-an-eye. Our progress over the past few thousand years may have been rapid and incredible, and has brought us to a point where we now seek to extend our civilization beyond Earth. We have come so far in such a short time, cosmically speaking, but whether we'll endure remains to be seen.
If we truly want to make any significant dent into "year 2" of the cosmic calendar, we have our work cut out for us. The world is changing rapidly and, on many fronts, we continue to damage and poison the ecosystem that sustains us. If we don't begin taking a longer-term view of our civilization, we could be gone in mere cosmic seconds, just as all of recorded human history fits into mere seconds as well. As the 2010s give way to the 2020s, it's up to all of us to pilot our one-and-only habitable planet, Earth, in the right direction.
Ethan Siegel - I am a Ph.D. astrophysicist, author, and science communicator, who professes physics and astronomy at various colleges. I have won numerous awards for science writing since 2008 for my blog, Starts With A Bang, including the award for best science blog by the Institute of Physics. My two books, Treknology: The Science of Star Trek from Tricorders to Warp Drive, Beyond the Galaxy: How humanity looked beyond our Milky Way and discovered the entire Universe, are available for purchase at Amazon. Follow me on Twitter @startswithabang.
Starts With A Bang Contributor Group
The Big Bang is widely accepted as being the beginning of everything we see around us, but other theories that are gathering support among scientists are suggesting otherwise.
The usual story of the Universe has a beginning, middle, and an end.
It began with the Big Bang 13.8 billion years ago when the Universe was tiny, hot, and dense. In less than a billionth of a billionth of a second, that pinpoint of a universe expanded to more than a billion, billion times its original size through a process called “cosmological inflation”.
Next came “the graceful exit”, when inflation stopped. The universe carried on expanding and cooling, but at a fraction of the initial rate. For the next 380,000 years, the Universe was so dense that not even light could move through it – the cosmos was an opaque, superhot plasma of scattered particles. When things finally cooled enough for the first hydrogen atoms to form, the Universe swiftly became transparent. Radiation burst out in every direction, and the Universe was on its way to becoming the lumpy entity we see today, with vast swaths of empty space punctuated by clumps of particles, dust, stars, black holes, galaxies, radiation, and other forms of matter and energy.
Eventually these lumps of matter will drift so far apart that they will slowly disappear, according to some models. The Universe will become a cold, uniform soup of isolated photons.
The Universe we can currently see is made up of clumps of particles, dust, stars, black holes, galaxies, radiation (Credit: NASA/JPL-Caltech/ESA/CXC/STScI)
It’s not a particularly dramatic ending, although it does have a satisfying finality.
But what if the Big Bang wasn’t actually the start of it all?
Perhaps the Big Bang was more of a “Big Bounce”, a turning point in an ongoing cycle of contraction and expansion. Or, it could be more like a point of reflection, with a mirror image of our universe expanding out the “other side”, where antimatter replaces matter, and time itself flows backwards. (There might even be a “mirror you” pondering what life looks like on this side.)
Perhaps the Big Bang was more of a “Big Bounce”, a turning point in an ongoing cycle of contraction and expansion
Or, the Big Bang might be a transition point in a universe that has always been – and always will be – expanding. All of these theories sit outside mainstream cosmology, but all are supported by influential scientists.
The growing number of these competing theories suggests that it might now be time to let go of the idea that the Big Bang marked the beginning of space and time. And, indeed, that it may even have an end.
Many competing Big Bang alternative stem from deep dissatisfaction with the idea of cosmological inflation.
Scars left by the Big Bang in a weak microwave radiation that permeates the entire cosmos provides clues about what the early Universe looked like (Credit: Nasa)
“I have to confess, I never liked inflation from the beginning,” says Neil Turok, the former director of the Perimeter Institute for Theoretical Physics in Waterloo, Canada.
“The inflationary paradigm has failed,” adds Paul Steinhardt, Albert Einstein professor in science at Princeton University, and proponent of a “Big Bounce” model.
“I always regarded inflation as a very artificial theory,” says Roger Penrose, emeritus Rouse Ball professor of mathematics at Oxford University. “The main reason that it didn't die at birth is that it was the only thing people could think of to explain what they call the ‘scale invariance of the Cosmic Microwave Background temperature fluctuations’.”
The Cosmic Microwave Background (or “CMB”) has been a fundamental factor in every model of the Universe since it was first observed in 1965. It’s a faint, ambient radiation found everywhere in the observable Universe that dates back to that moment when the Universe first became transparent to radiation.
The CMB is a major source of information about what the early Universe looked like. It is also a tantalising mystery for physicists. In every direction scientists point a radio telescope, the CMB looks the same, even in regions that seemingly could never have interacted with one another at any point in the history of a 13.8 billion-year- old universe.
Our observable universe expanded from one tiny homogenous region within that primordial hot mess
“The CMB temperature is the same on opposite sides of the sky and those parts of the sky would never have been in causal contact,” says Katie Mack, a cosmologist at North Carolina State University. “Something had to connect those two regions of the Universe in the past. Something had to tell that part of the sky to be the same temperature as that part of the sky.”
Without some mechanism to even out the temperature across the observable Universe, scientists would expect to see much larger variations in different regions.
Inflation offers a way to solve this so-called “homogeneity problem”. With a period of insane expansion stretching out the Universe so rapidly that almost the entire thing ended up far beyond the region we can observe and interact with. Our observableuniverse expanded from one tiny homogenous region within that primordial hot mess, producing the uniform CMB. Other regions beyond what we can observe might look very different.
Theoretical physicists are increasingly finding that inflation theory fails to account for the spread of matter and energy observed in the Universe (Credit: Nasa/ESA)
“Inflation seems to be the thing that has enough support from the data that we can take it as the default,” says Mack. ”It's the one I teach in my classes. But I always say that we don't know for sure that this happened. But it seems to fit the data pretty well, and is what most people would say is most likely.”
But there have always been shortcomings with the theory. Notably, there is no definitive mechanism to trigger inflationary expansion, or a testable explanation for how the graceful ending could happen. One idea put forward by proponents of inflation is that theoretical particles made up something called an “inflation field” that drove inflation and then decayed into the particles we see around us today.
But even with tweaks like this, inflation makes predictions that have, at least thus far, not been confirmed. The theory says spacetime should be warped by primordial gravitational waves that ricocheted out across the Universe with the Big Bang. But while certain types of gravitational waves have been detected, none of these primordial ones have yet been found to support the theory.
Quantum physics also forces inflation theories into very messy territory. Rare quantum fluctuations are predicted to cause inflation to break space up into an infinite number of patches with wildly different properties – a “multiverse” in which literally every imaginable outcome occurs.
“The theory is completely indecisive,” says Steinhardt. “It can only say that the observable Universe might be like this or that or any other possibility you can imagine, depending on where we happen to be in the multiverse. Nothing is ruled out that is physically conceivable.”
Steinhardt, who was one of the original architects of inflationary theory, ultimately got fed up with the lack of predictiveness and untestability.
“Do we really need to imagine that there exist an infinite number of messy universes that we have never seen and never will see in order to explain the one simple and remarkably smooth Universe we actually observe?” he asks. “I say no. We have to look for a better idea.”
Rather than being a beginning, the Big Bang could have been a moment of transition from one period of space and time to another – more of a bounce (Credit: Alamy)
The problem might have to do with the Big Bang itself, and with the idea that there was a beginning to space and time.
The “Big Bounce” theory agrees with the Big Bang picture of a hot, dense universe 13.8 billion years ago that began to expand and cool. But rather than being the beginning of space and time, that was a moment of transition from an earlier phase during which space was contracting.
With a bounce rather than a bang, Steinhardt says, distant parts of the cosmos would have plenty of time to interact with each other, and to form a single smooth universe in which the sources of CMB radiation would have had a chance to even out.
In fact, it’s possible that time has existed forever.
“And if a bounce happened in our past, why could there not have been many of them?” says Steinhardt. “In that case, it is plausible that there is one in our future. Our expanding universe could start to contract, returning to that dense state and starting the bounce cycle again.”
Steinhardt and Turok worked together on some early versions of the Big Bounce model, in which the Universe shrunk to such a tiny size that quantum physics took over from classical physics, leaving the predictions uncertain. But more recently, another of Steinhardt’s collaborators, Anna Ijjas, developed a model in which the Universe never gets so small that quantum physics dominates.
“It’s a rather prosaic, conservative idea described at all times by classical equations,” Steinhardt says. “Inflation says there’s a multiverse, that there’s an infinite number of ways the Universe might come out, and we just happen to live in the one that is smooth and flat. That’s possible but not likely. This Big Bounce model says this is how the Universe must be.”
Neil Turok has also been exploring another avenue for a simpler alternative to inflationary theory, the “Mirror Universe”. It predicts that another universe dominated by antimatter, but governed by the same physical laws as our own, is expanding outwards on the other side of the Big Bang – a kind of “anti-universe”, if you like.
“I take one thing away from the observations of the last 30 years, which is that the Universe is unbelievably simple,” he says. “At large scales, it is not chaotic. It is not random. It's incredibly ordered and regular and requires very few numbers to describe everything.”
Our forward-time flowing universe could have a perfect reflection that also extends out in reverse from the event we call the Big Bang (Credit: Alamy)
With this in mind, Turok sees no place for a multiverse, higher dimensions, or new particles to explain what can be seen when we look up at the heavens. The Mirror Universe offers all that – and might also solve one of the Universe’s big mysteries.
If you add up all the known mass in a galaxy – stars, nebulae, black holes and so on – the total doesn’t create enough gravity to explain the motion within and between galaxies. The remainder seems to be made up of something we cannot currently see – dark matter. This mysterious stuff accounts for about 85% of the matter in the universe.
The Mirror Universe model predicts that the Big Bang produced a particle known as “right-handed neutrinos” in abundance. While particle physicists have yet to directly see any of these particles, they are pretty sure they exist. And it is these that make up dark matter, according to those who support the Mirror Universe theory.
“It’s the only particle on that list (of particles in the Standard Model) that has the two requisite properties that we haven't directly observed it yet, and it could be stable,” says Latham Boyle, another leading proponent of the Mirror Universe theory and a colleague of Turok at the Perimeter Institute.
The entire picture of what we know nowadays, the whole history of the Universe, is what I call one ‘aeon’ in a succession of aeons – Roger Penrose
Perhaps the most challenging alternative to the Big Bang and inflation is Roger Penrose’s “Conformal Cyclic Cosmology” theory (CCC). Like the Big Bounce, it involves a universe that might have existed forever. But in CCC, it never goes through a period of contraction – it only ever expands.
“The view I have is that the Big Bang was not the beginning,” says Penrose. “The entire picture of what we know nowadays, the whole history of the Universe, is what I call one ‘aeon’ in a succession of aeons.”
Penrose’s model predicts that much of the matter in the Universe will eventually be dragged into ultra-massive black holes. As the Universe expands and cools to near absolute zero, those black holes will “boil away” through a phenomenon called Hawking Radiation.
“You have to think in terms of something like a googol years, which means a number one with 100 zeros,” says Penrose. “That’s the number of years or more for the really big ones to finally evaporate away. And then you’ve got a universe really dominated by photons (particles of light).”
Penrose says at this point, the Universe begins to look much as it did at its start, setting the stage for the start of another aeon.
Conformal Cyclic Cosmology predicts that much of the Universe will be pulled into enormous black holes that will then boil away (Credit: NASA/JPL-Caltech)
One of the predictions of CCC is that there might be a record of the previous aeon in the cosmic microwave background radiation that originally inspired the inflation model. When hyper-massive black holes collide, the impact creates a huge release of energy in the form of gravitational waves. When giant black holes finally evaporate, they release a huge amount of energy in the form of low-frequency photons. Both of these phenomena are so powerful, Penrose says, that they can “burst through to the other side” of a transition from one aeon to the next, each leaving its own kind of “signal” embedded in the CMB like an echo from the past.
Penrose calls the patterns left behind by evaporating black holes “Hawking Points”.
For the first 380,000 years of the current aeon, these would have been nothing more than tiny points in the cosmos, but as the Universe has expanded, they would appear as “splotches” across the sky.
Penrose has been working with Polish, Korean and Armenian cosmologists to see if these patterns can actually be found by comparing measurements of the CMB with thousands of random patterns.
“The conclusion we come to is that we see these spots in the sky with 99.98% confidence,” Penrose says. The physics world has, however, remained largely skeptical of these results to date and there has been limited interest among cosmologists about even attempting to replicate Penrose’s analysis.
It is unlikely that we will ever be able to directly observe what happened in the first moments after the Big Bang, let alone the moments before. The opaque superheated plasma that existed in the early moments will likely forever obscure our view. But there are other potentially observable phenomena such as primordial gravitational waves, primordial black holes, right-handed neutrinos, that could provide us some clues about which of the theories about our universe are correct.
“As we develop new theories and new models of cosmology, those will give us other interesting predictions that can that we can look for,” says Mack. “The hope is not necessarily that we're going to see the beginning more directly, but that maybe through some roundabout way we'll better understand the structure of physics itself.”
Until then, the story of our universe, its beginnings and whether it has an end, will continue to be debated.