An illustration of our cosmic history, from the Big Bang until the present, within the context of the expanding Universe. We cannot be certain, despite what many have contended, that the Universe began from a singularity. We can, however, break the illustration you see into the different eras based on properties the Universe had at those particular times. We are already in the Universe’s 6th and final era.
NASA / WMAP science team
The Universe is not the same today as it was yesterday. With each moment that goes by, a number of subtle but important changes occur, even if many of them are imperceptible on measurable, human timescales. The Universe is expanding, which means that the distances between the largest cosmic structures are increasing with time.
A second ago, the Universe was slightly smaller; a second from now, the Universe will be slightly larger. But those subtle changes both build up over large, cosmic timescales, and affect more than just distances. As the Universe expands, the relative importance of radiation, matter, neutrinos, and dark energy all change. The temperature of the Universe changes. And what you’d see in the sky would change dramatically as well. All told, there are six different eras we can break the Universe into, and we’re already in the final one.
The reason for this can be understood from the graph above. Everything that exists in our Universe has a certain amount of energy in it: matter, radiation, dark energy, etc. As the Universe expands, the volume that these forms of energy occupy changes, and each one will have its energy density evolve differently. In particular, if we define the observable horizon by the variable a, then:
matter will have its energy density evolve as 1/a3, since (for matter) density is just mass over volume, and mass can easily be converted to energy via E = mc2,
radiation will have its energy density evolve as 1/a4, since (for radiation) the number density is the number of particles divided by volume, and the energy of each individual photon stretches as the Universe expands, adding an additional factor of 1/a relative to matter,
and dark energy is a property of space itself, so its energy density remains constant (1/a0), irrespective of the Universe’s expansion or volume.
A Universe that has been around longer, therefore, will have expanded more. It will be cooler in the future and was hotter in the past; it was gravitationally more uniform in the past and is clumpier now; it was smaller in the past and will be much, much larger in the future.
By applying the laws of physics to the Universe, and comparing the possible solutions with the observations and measurements we’ve obtained, we can determine both where we came from and where we’re headed. We can extrapolate our past history all the way back to the beginning of the hot Big Bang and even before, to a period of . We can extrapolate our current Universe into the far distant future as well, and foresee the ultimate fate that awaits everything that exists.
When we draw the dividing lines based on how the Universe behaves, we find that there are six different eras that will come to pass.
Inflationary era: which preceded and set up the hot Big Bang.
Primordial Soup era: from the start of the hot Big Bang until the final transformative nuclear & particle interactions occur in the early Universe.
Plasma era: from the end of non-scattering nuclear and particle interactions until the Universe cools enough to stably form neutral matter.
Dark Ages era: from the formation of neutral matter until the first stars and galaxies reionize the intergalactic medium of the Universe completely.
Stellar era: from the end of reionization until the gravity-driven formation and growth of large-scale structure ceases, when the dark energy density dominates over the matter density.
Dark Energy era: the final stage of our Universe, where the expansion accelerates and disconnected objects speed irrevocably and irreversibly away from one another.
We already entered this final era billions of years ago. Most of the important events that will define our Universe’s history have already occurred.
1.) Inflationary era. Prior to the hot Big Bang, the Universe wasn’t filled with matter, antimatter, dark matter or radiation. It wasn’t filled with particles of any type. Instead, it was filled with a form of energy inherent to space itself: a form of energy that caused the Universe to expand both extremely rapidly and relentlessly, in an exponential fashion.
It stretched the Universe, from whatever geometry it once had, into a state indistinguishable from spatially flat.
It expanded a small, causally connected patch of the Universe to one much larger than our presently visible Universe: larger than the current causal horizon.
It took any particles that may have been present and expanded the Universe so rapidly that none of them are left inside a region the size of our visible Universe.
And the quantum fluctuations that occurred during inflation created the seeds of structure that gave rise to our vast cosmic web today.
And then, abruptly, some 13.8 billion years ago, inflation ended. All of that energy, once inherent to space itself, got converted into particles, antiparticles, and radiation. With this transition, the inflationary era ended, and the hot Big Bang began.
2.) Primordial Soup era. Once the expanding Universe is filled with matter, antimatter and radiation, it’s going to cool. Whenever particles collide, they’ll produce whatever particle-antiparticle pairs are allowed by the laws of physics. The primary restriction comes only from the energies of the collisions involved, as the production is governed by E = mc2.
As the Universe cools, the energy drops, and it becomes harder and harder to create more massive particle-antiparticle pairs, but annihilations and other particle reactions continue unabated. 1-to-3 seconds after the Big Bang, the antimatter is all gone, leaving only matter behind. 3-to-4 minutes after the Big Bang, stable deuterium can form, and nucleosynthesis of the light elements occurs. And after some radioactive decays and a few final nuclear reactions, all we have left is a hot (but cooling) ionized plasma consisting of photons, neutrinos, atomic nuclei and electrons.
3.) Plasma era. Once those light nuclei form, they’re the only positively (electrically) charged objects in the Universe, and they’re everywhere. Of course, they’re balanced by an equal amount of negative charge in the form of electrons. Nuclei and electrons form atoms, and so it might seem only natural that these two species of particle would find one another immediately, forming atoms and paving the way for stars.
Unfortunately for them, they’re vastly outnumbered — by more than a billion to one — by photons. Every time an electron and a nucleus bind together, a high-enough energy photon comes along and blasts them apart. It isn’t until the Universe cools dramatically, from billions of degrees to just thousands of degrees, that neutral atoms can finally form. (And even then, it’s only possible because of a special atomic transition.)
At the beginning of the Plasma era, the Universe’s energy content is dominated by radiation. By the end, it’s dominated by normal and dark matter. This third phase takes us to 380,000 years after the Big Bang.
S. G. Djorgovski et al., Caltech Digital Media Center
4.) Dark Ages era. Filled with neutral atoms, at last, gravitation can begin the process of forming structure in the Universe. But with all these neutral atoms around, what we presently know as visible light would be invisible all throughout the sky.
Why’s that? Because neutral atoms, particularly in the form of cosmic dust, are outstanding at blocking visible light.
In order to end these dark ages, the intergalactic medium needs to be reionized. That requires enormous amounts of star-formation and tremendous numbers of ultraviolet photons, and that requires time, gravitation, and the start of the cosmic web. The first major regions of reionization take place 200-250 million years after the Big Bang, but reionization doesn’t complete, on average, until the Universe is 550 million years old. At this point, the star-formation rate is still increasing, and the first massive galaxy clusters are just beginning to form.
NASA, ESA, A. Koekemoer (STScI), M. Jauzac (Durham University), C. Steinhardt (Niels Bohr Institute), and the BUFFALO team
5.) Stellar era. Once the dark ages are over, the Universe is now transparent to starlight. The great recesses of the cosmos are now accessible, with stars, star clusters, galaxies, galaxy clusters, and the great, growing cosmic web all waiting to be discovered. The Universe is dominated, energy-wise, by dark matter and normal matter, and the gravitationally bound structures continue to grow larger and larger.
The star-formation rate rises and rises, peaking about 3 billion years after the Big Bang. At this point, new galaxies continue to form, existing galaxies continue to grow and merge, and galaxy clusters attract more and more matter into them. But the amount of free gas within galaxies begins to drop, as the enormous amounts of star-formation have used up a large amount of it. Slowly but steadily, the star-formation rate drops.
As time goes forward, the stellar death rate will outpace the birth rate, a fact made worse by the following surprise: as the matter density drops with the expanding Universe, a new form of energy — dark energy — begins to appear and dominate. 7.8 billion years after the Big Bang, distant galaxies stop slowing down in their recession from one another, and begin speeding up again. The accelerating Universe is upon us. A little bit later, 9.2 billion years after the Big Bang, dark energy becomes the dominant component of energy in the Universe. At this point, we enter the final era.
NASA & ESA
6.) Dark Energy age. Once dark energy takes over, something bizarre happens: the large-scale structure in the Universe ceases to grow. The objects that were gravitationally bound to one another before dark energy’s takeover will remain bound, but those that were not yet bound by the onset of the dark energy age will never become bound. Instead, they will simply accelerate away from one another, leading lonely existences in the great expanse of nothingness.
The individual bound structures, like galaxies and groups/clusters of galaxies, will eventually merge to form one giant elliptical galaxy. The existing stars will die; new star formation will slow down to a trickle and then stop; gravitational interactions will eject most of the stars into the intergalactic abyss. Planets will spiral into their parent stars or stellar remnants, owing to decay by gravitational radiation. Even black holes, with extraordinarily long lifetimes, will eventually decay from Hawking radiation.
Image courtesy of Jeff Bryant
In the end, only black dwarf stars and isolated masses to small to ignite nuclear fusion will remain, sparsely populated and disconnected from one another in this empty, ever-expanding cosmos. These final-state corpses will exist even googols of years onward, continuing to persist as dark energy remains the dominant factor in our Universe.
This last era, of dark energy domination, has already begun. Dark energy became important for the Universe’s expansion 6 billion years ago, and began dominating the Universe’s energy content around the time our Sun and Solar System were being born. The Universe may have six unique stages, but for the entirety of Earth’s history, we’ve already been in the final one. Take a good look at the Universe around us. It will never be this rich — or this easy to access — ever again.
In 1915, Einstein’s theory of General Relativity gave us a brand new theory of gravity, based on the geometrical concept of curved spacetime. Matter and energy told space how to curve; curved space told matter and energy how to move. By 1922, scientists had discovered that if you fill the Universe uniformly with matter and energy, it won’t remain static, but will either expand or contract. By the end of the 1920s, led by the observations of Edwin Hubble, we had discovered our Universe was expanding, and had our first measurement of the expansion rate.
The journey to pin down exactly what that rate is has now hit a snag, with two different measurement techniques yielding inconsistent results. It could be an indicator of new physics. But there could be an even simpler solution, and nobody wants to talk about it.
Standard candles (L) and standard rulers (R) are two different techniques astronomers use to measure the expansion of space at various times/distances in the past. Based on how quantities like luminosity or angular size change with distance, we can infer the expansion history of the Universe.NASA / JPL-Caltech
The controversy is as follows: when we see a distant galaxy, we’re seeing it as it was in the past. But it isn’t simply that you look at light that took a billion years to arrive and conclude that the galaxy is a billion light years away. Instead, the galaxy will actually be more distant than that.
Why’s that? Because the space that makes up our Universe itself is expanding. This prediction of Einstein’s General Relativity, first recognized in the 1920s and then observationally validated by Edwin Hubble several years later, has been one of the cornerstones of modern cosmology.
A plot of the apparent expansion rate (y-axis) vs. distance (x-axis) is consistent with a Universe that expanded faster in the past, but where distant galaxies are accelerating in their recession today. This is a modern version of, extending thousands of times farther than, Hubble’s original work. Note the fact that the points do not form a straight line, indicating the expansion rate’s change over time.Ned Wright, based on the latest data from Betoule et al. (2014)
The big question is how to measure it. How do we measure how the Universe is expanding? All methods invariably rely on the same general rules:
you pick a point in the Universe’s past where you can make an observation,
you measure the properties you can measure about that distant point,
and you calculate how the Universe would have had to expand from then until now to reproduce what you see.
This could be from a wide variety of methods, ranging from observations of the nearby Universe to objects billions of light years away.
The Planck satellite’s data, combined with the other complementary data suites, gives us very tight constraints on the allowed values of cosmological parameters. The Hubble expansion rate today, in particular, is tightly constrained to be between 67 and 68 km/s/Mpc, with very little wiggle-room. The measurements from the Cosmic Distance Ladder method (Riess et al., 2018) are not consistent with this result.PLANCK 2018 RESULTS. VI. COSMOLOGICAL PARAMETERS; PLANCK COLLABORATION (2018)
For many years now, there’s been a controversy brewing. Two different measurement methods — one using the cosmic distance ladder and one using the first observable light in the Universe — give results that are mutually inconsistent. The tension has enormous implications that something may be wrong with how we conceive of the Universe.
There is another explanation, however, that’s much simpler than the idea that either something is wrong with the Universe or that some new physics is required. Instead, it’s possible that one (or more) method has a systematic error associated with it: an inherent flaw to the method that hasn’t been identified yet that’s biasing its results. Either method (or even both methods) could be at fault. Here’s the story of how.
The Variable Star RS Puppis, with its light echoes shining through the interstellar clouds. Variable stars come in many varieties; one of them, Cepheid variables, can be measured both within our own galaxy and in galaxies up to 50-60 million light years away. This enables us to extrapolate distances from our own galaxy to far more distant ones in the Universe.NASA, ESA, and the Hubble Heritage Team
The cosmic distance ladder is the oldest method we have to compute the distances to faraway objects. You start by measuring something close by: the distance to the Sun, for example. Then you use direct measurements of distant stars using the motion of the Earth around the Sun — known as parallax — to calculate the distance to nearby stars. Some of these nearby stars will include variable stars like Cepheids, which can be measured accurately in nearby and distant galaxies, and some of those galaxies will contain events like type Ia supernovae, which are some of the most distant objects of all.
Make all of these measurements, and you can derive distances to galaxies many billions of light years away. Put it all together with easily-measurable redshifts, and you’ll arrive at a measurement for the rate of expansion of the Universe.
The construction of the cosmic distance ladder involves going from our Solar System to the stars to nearby galaxies to distant ones. Each “step” carries along its own uncertainties, especially the Cepheid variable and supernovae steps; it also would be biased towards higher or lower values if we lived in an underdense or overdense region.NASA, ESA, A. FEILD (STSCI), AND A. RIESS (STSCI/JHU)
This is how dark energy was first discovered, and our best methods of the cosmic distance ladder give us an expansion rate of 73.2 km/s/Mpc, with an uncertainty of less than 3%.
Universal light-curve properties for Type Ia supernovae. This result, first obtained in the late 1990s, has recently been called into question; supernovae may not. in fact, have light curves that are as universal as previously thought.S. Blondin and Max Stritzinger
On the other hand, we have measurements of the Universe’s composition and expansion rate from the earliest available picture of it: the Cosmic Microwave Background. The minuscule, 1-part-in-30,000 temperature fluctuations display a very specific pattern on all scales, from the largest all-sky ones down to 0.07° or so, where its resolution is limited by the fundamental astrophysics of the Universe itself.
The final results from the Planck collaboration show an extraordinary agreement between the predictions of a dark energy/dark matter-rich cosmology (blue line) with the data (red points, black error bars) from the Planck team. All 7 acoustic peaks fit the data extraordinarily well.PLANCK 2018 RESULTS. VI. COSMOLOGICAL PARAMETERS; PLANCK COLLABORATION (2018)
Based on the full suite of data from Planck, we have exquisite measurements for what the Universe is made of and how it’s expanded over its history. The Universe is 31.5% matter (where 4.9% is normal matter and the rest is dark matter), 68.5% dark energy, and just 0.01% radiation. The Hubble expansion rate, today, is determined to be 67.4 km/s/Mpc, with an uncertainty of only around 1%. This creates an enormous tension with the cosmic distance ladder results.
An illustration of clustering patterns due to Baryon Acoustic Oscillations, where the likelihood of finding a galaxy at a certain distance from any other galaxy is governed by the relationship between dark matter and normal matter. As the Universe expands, this characteristic distance expands as well, allowing us to measure the Hubble constant, the dark matter density, and even the scalar spectral index. The results agree with the CMB data.ZOSIA ROSTOMIAN
In addition, we have another measurement from the distant Universe that gives another measurement, based on the way that galaxies cluster together on large scales. When you have a galaxy, you can ask a simple-sounding question: what is the probability of finding another galaxy a specific distance away?
Based on what we know about dark matter and normal matter, there’s an enhanced probability of finding a galaxy 500 million light years distant from another versus 400 million or 600 million. This is for today, and so as the Universe was smaller in the past, the distance scale corresponding to this probability enhancement changes as the Universe expands. This method is known as the inverse distance ladder, and gives a third method to measure the expanding Universe. It also gives an expansion rate of around 67 km/s/Mpc, again with a small uncertainty.
Modern measurement tensions from the distance ladder (red) with CMB (green) and BAO (blue) data. The red points are from the distance ladder method; the green and blue are from ‘leftover relic’ methods. Note that the errors on red vs. green/blue measurements do not overlap.AUBOURG, ÉRIC ET AL. PHYS.REV. D92 (2015) NO.12, 123516.
Now, it’s possible that both of these measurements have a flaw in them, too. In particular, many of these parameters are related, meaning that if you try and increase one, you have to decrease-or-increase others. While the data from Planck indicates a Hubble expansion rate of 67.4 km/s/Mpc, that rate could be higher, like 72 km/s/Mpc. If it were, that would simply mean we needed a smaller amount of matter (26% instead of 31.5%), a larger amount of dark energy (74% instead of 68.5%), and a larger scalar spectral index (ns) to characterize the density fluctuations (0.99 instead of 0.96).
This is deemed highly unlikely, but it illustrates how one small flaw, if we overlooked something, could keep these independent measurements from aligning.
Before Planck, the best-fit to the data indicated a Hubble parameter of approximately 71 km/s/Mpc, but a value of approximately 70 or above would now be too great for both the dark matter density (x-axis) we’ve seen via other means and the scalar spectral index (right side of the y-axis) that we require for the large-scale structure of the Universe to make sense.P.A.R. ADE ET AL. AND THE PLANCK COLLABORATION (2015)
There are a lot of problems that arise for cosmology if the teams measuring the Cosmic Microwave Background and the inverse distance ladder are wrong. The Universe, from the measurements we have today, should not have the low dark matter density or the high scalar spectral index that a large Hubble constant would imply. If the value truly is closer to 73 km/s/Mpc, we may be headed for a cosmic revolution.
Correlations between certain aspects of the magnitude of temperature fluctuations (y-axis) as a function of decreasing angular scale (x-axis) show a Universe that is consistent with a scalar spectral index of 0.96 or 0.97, but not 0.99 or 1.00.P.A.R. ADE ET AL. AND THE PLANCK COLLABORATION
On the other hand, if the cosmic distance ladder team is wrong, owing to a fault in any rung on the distance ladder, the crisis is completely evaded. There was one overlooked systematic, and once it’s resolved, every piece of the cosmic puzzle falls perfectly into place. Perhaps the value of the Hubble expansion rate really is somewhere between 66.5 and 68 km/s/Mpc, and all we had to do was identify one astronomical flaw to get there.
The fluctuations in the CMB, the formation and correlations between large-scale structure, and modern observations of gravitational lensing, among many others, all point towards the same picture: an accelerating Universe, containing and full of dark matter and dark energy.Chris Blake and Sam Moorfield
The possibility of needing to overhaul many of the most compelling conclusions we’ve reached over the past two decades is fascinating, and is worth investigating to the fullest. Both groups may be right, and there may be a physical reason why the nearby measurements are skewed relative to the more distant ones. Both groups may be wrong; they may both have erred.
But this controversy could end with the astronomical equivalent of a loose OPERA cable. The distance ladder group could have a flaw, and our large-scale cosmological measurements could be as good as gold. That would be the simplest solution to this fascinating saga. But until the critical data comes in, we simply don’t know. Meanwhile, our scientific curiosity demands that we investigate. No less than the entire Universe is at stake.