The Universe is already in its sixth and final era
From before the Big Bang to the present day, the Universe goes through many eras. Dark energy heralds the final one.
The Universe proceeded through many significant stages in its existence, from cosmic expansion to a primordial particle soup to the expanding, cooling aftermath. However, some 6 billion years ago, a new type of energy began to dominate the expansion of the Universe: dark energy, which today controls our cosmic fate. The era in which dark energy dominates the expansion of the Universe is the last one that our Universe will ever see. Here’s why we’re already witnessing the beginning of the end.
The Universe is no longer the same as it was yesterday. A number of small but significant changes occur with each passing instant, even if many of them are unnoticeable on measurable, human timescales. Because the Universe is expanding, the distances between the biggest cosmic structures are rising over time.
The Universe was little smaller a second ago; it will be slightly greater a second from now. However, such minute changes accumulate over vast cosmic durations and effect much than simply distances. The relative role of radiation, matter, neutrinos, and dark energy changes as the Universe expands. The Universe’s temperature changes. What you see in the sky would also alter substantially. There are six separate eras that we may divide the Universe into, and we are currently living in the final one.
While matter (both normal and dark) and radiation become less dense as the Universe expands due to its rising volume, dark energy, as well as field energy during inflation, is a type of energy that exists in space itself. The density of dark energy remains constant as new space is created in the expanding Universe. (Credit: E. Siegel/Beyond the Galaxy)
The explanation for this can be seen in the graph above. Everything in our Universe has energy in some form or another: matter, radiation, dark energy, and so on. The volume that these types of energy occupy changes as the Universe expands, and their energy density evolves differently. In specifically, if the visible horizon is defined by the variable a, then:
- Because density (for matter) is simply mass over volume, and mass can easily be transformed to energy via E = mc2, matter’s energy density will evolve as 1/a3
- Because the number density of radiation is the number of particles divided by volume, and the energy of each individual photon extends as the Universe expands, radiation’s energy density will evolve as 1/a4, adding an additional factor of 1/a compared to matter.
- Because dark energy is a property of space, its energy density (1/a0) remains constant regardless of the Universe’s expansion or volume.
A graphic history of the expanding Universe encompasses the hot, dense condition known as the Big Bang, as well as the subsequent growth and formation of structure. The whole set of facts, including light element observations and the cosmic microwave background, leaves only the Big Bang as a viable explanation for everything we see. The Universe cools as it expands, allowing ions, neutral atoms, and eventually molecules, gas clouds, stars, and galaxies to form. (Credit: NASA/CXC/M. Weiss)
As a result, a Universe that has been around for a longer time will have expanded more. It will be cooler in the future and hotter in the past; it was more uniform gravitationally in the past and is now clumpier; it was smaller in the past and will be much, much larger in the future.
We can identify where we came from and where we’re going by applying the rules of physics to the Universe and comparing the various solutions to the observations and measurements we’ve acquired. We can trace our origins all the way back to the hot Big Bang and even before that, to a period of cosmic inflation. We can extrapolate our current Universe into the far distant future as well, and foresee the ultimate fate that awaits everything that exists.
Our entire cosmic history is theoretically fully understood, but only because we understand the underlying theory of gravitation and know the Universe’s current expansion rate and energy composition. Light will always propagate across this expanding Universe, and we will continue to receive it arbitrarily far into the future, but it will be restricted in time by what reaches us. To see the things that are currently visible, we will need to explore to fainter brightnesses and longer wavelengths, but these are technological rather than physical restrictions. (Credit: Nicole Rager Fuller/National Science Foundation)
When we draw the dividing lines based on how the Universe behaves, we discover that there will be six distinct periods.
- 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.
This final epoch began billions of years ago. The majority of the significant events that will define the history of our Universe have already occurred.
When inflation finishes, the quantum fluctuations that occur are extended across the Universe and become density fluctuations. This results in the large-scale structure of the Universe we see today, as well as the temperature changes seen in the CMB. It’s a remarkable demonstration of how reality’s quantum nature influences the entire large-scale universe. (Credit: E. Siegel; ESA/Planck and the DOE/NASA/NSF Interagency Task Force on CMB research)
1.) Inflationary era. The Universe was empty of matter, antimatter, dark matter, or radiation before to the hot Big Bang. It was empty of any kind of particle. Instead, it was filled with a type of energy inherent in space itself: energy that allowed the Universe to grow in an exponential fashion, both exceedingly quickly and relentlessly.
- It stretched the Universe from whatever geometry it formerly had to an indistinguishable state from spatial flat.
- It grew a small, causally connected region of the Universe into something much larger than our current observable Universe: larger than the current causal horizon.
- It took any particles that were present and expanded the Universe so quickly that none of them remained within a region the size of our visible Universe.
- And the quantum fluctuations that happened during inflation formed the structural seeds that gave life to our huge cosmic network today.
Then, suddenly, 13.8 billion years ago, inflation ended. All of the energy that was formerly inherent in space was turned into particles, antiparticles, and radiation. The inflationary era ended with this shift, and the hot Big Bang began.
Given enough energy, the very young Universe’s high temperatures can produce not only particles and photons, but also antiparticles and unstable particles, resulting in a primordial particle-and-antiparticle soup. Even under these conditions, however, only a few distinct states or particles can emerge. (Credit: Brookhaven National Laboratory)
2.) Primordial Soup era. The expanding Universe will cool once it is full of matter, antimatter, and radiation. When particles collide, they form any particle-antiparticle pairings the laws of physics allow. Because the production is governed by E = mc2, the basic constraint is just the energy of the collisions involved.
The energy in the Universe decreases as it cools, making it more difficult to form more large particle-antiparticle pairs, although annihilations and other particle processes continue unabated. One to three seconds after the Big Bang, all antimatter has vanished, leaving just matter. Stable deuterium can form three to four minutes after the Big Bang, and nucleosynthesis of the light elements occurs. After a few radioactive decays and a few final nuclear events, all that remains is a hot (but cooling) ionized plasma made up of photons, neutrinos, atomic nuclei, and electrons.
Initially (left), photons scatter off of electrons and have enough energy to return any atoms to an ionized state. Once the Universe has cooled sufficiently and is devoid of such high-energy photons (right), they are unable to interact with neutral atoms and instead merely free-stream, because they have the wrong wavelength to excite these atoms to a higher energy level. (Credit: E. Siegel/Beyond the Galaxy)
3.) Plasma era. They are the only positively (electrically) charged objects in the Universe once those light nuclei develop, and they are everywhere. Of course, an equal quantity of negative charge in the form of electrons balances them out. Atoms are formed by nuclei and electrons, thus it seems obvious that these two types of particles would collide and produce atoms, opening the way for stars.
Unfortunately for them, photons outnumber them by more than a billion to one. When an electron and a nucleus bind together, a photon with sufficient energy comes along and blasts them apart. Neutral atoms cannot form until the Universe cools considerably, from billions of degrees to thousands of degrees. (And even then, it’s only possible because of a special atomic transition.)
Radiation dominates the Universe’s energy content at the start of the Plasma period. By the end, normal and dark matter had taken over. This third phase occurs 380,000 years after the Big Bang.
Diagram illustrating the history of the Universe, highlighting reionization. The Universe was full with light-blocking, neutral atoms before stars or galaxies emerged. While the majority of the Universe does not fully reionized until 550 million years later, some places achieve full reionization sooner than others. The first large waves of reionization occur around 250 million years ago, whereas a few lucky stars may develop only 50-100 million years after the Big Bang. We may be able to learn more about the universe with the correct tools, such as the James Webb Space Telescope. (Credit: S. G. Djorgovski et al., Caltech. Produced with the help of the Caltech Digital Media Center)
4.) Dark Ages era. Finally, with the Universe filled with neutral atoms, gravitation can begin the process of generating structure. However, with all of these neutral atoms in the sky, what we now call visible light would be completely invisible.
Why? Because neutral atoms, particularly cosmic dust, are excellent at obscuring visible light.
The intergalactic medium must be reionized to bring these dark eras to an end. That necessitates massive volumes of star formation and massive numbers of ultraviolet photons, as well as time, gravitation, and the beginning of the cosmic web. The first large regions of reionization occur 200 to 250 million years after the Big Bang, but reionization does not finish until the Universe is 550 million years old on average. At this moment, the pace of star creation is still accelerating, and the first enormous galaxy clusters are forming.
Abell 370, shown here, is one of six enormous galaxy clusters imaged by the Hubble Frontier Fields study. Thousands of ultra-distant galaxies were uncovered since other great observatories were also employed to examine this region of sky. Hubble’s BUFFALO (Beyond Ultra-deep Frontier Fields And Legacy Observations) program will get distances to these galaxies by observing them again with a new scientific purpose, allowing us to better understand how galaxies started, evolved, and grew up in our Universe. When combined with intracluster light measurements, we might acquire a better picture of the dark matter inside through several lines of evidence of the same structure. (Credit: 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 becomes 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 rate of star formation continues to increase, reaching a peak roughly 3 billion years after the Big Bang. At this point, new galaxies are forming, existing galaxies are expanding and merging, and galaxy clusters are attracting more and more matter. However, the amount of free gas within galaxies begins to decrease as massive volumes of star formation consumes a large portion of it. The pace of star production is gradually but steadily decreasing.
As time passes, the stellar death rate will outstrip the stellar birth rate, which will be worsened by the following surprise: As the density of matter decreases with the expansion of the Universe, a new type of energy – dark energy — emerges and takes over. About 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 speeding Universe has arrived. Dark energy becomes the primary component of energy in the Universe 9.2 billion years after the Big Bang. We have now entered the final phase.
The various conceivable outcomes of the Universe, with our actual, accelerating fate represented on the right. After enough time has passed, the acceleration will isolate every bound galaxy or supergalactic structure in the Universe, as all other structures accelerate irrevocably away. We can only infer the presence and qualities of dark energy from the past, which need at least one constant, but the consequences for the future are far-reaching. (Credit: NASA & ESA)
6.) Dark Energy age. When dark energy takes over, something strange happens: the Universe’s large-scale structure stops growing. Objects that were gravitationally bound to one another before to the takeover by dark energy will remain bound, while those that were not bound at the start of the dark energy age will never be bound. Instead, they’ll simply speed apart from each other, leading lonely lives in the large expanse of nothingness.
Individual bound structures, like as galaxies and groups/clusters of galaxies, will eventually combine to form a single massive elliptical galaxy. Existing stars will die; new star formation will slow to a trickle and finally stop; and gravitational interactions will eject the vast majority of stars into the intergalactic abyss. Due to gravitational radiation decay, planets will drift into their parent stars or stellar remnants. Even black holes, which have extremely lengthy lifetimes, will eventually decay due to Hawking radiation.
If nothing ejects or collides with the leftovers of Earth after the sun becomes a black dwarf, gravitational radiation will gradually cause us to spiral in, be torn apart, and eventually swallowed by the remnant of our sun. (Credit: Jeff Bryant/Vistapro)
Only black dwarf stars and isolated masses too tiny to ignite nuclear fusion will survive in this barren, ever-expanding cosmos, sparsely populated and isolated from one another. These final-state corpses will continue to exist for googols of years as long as dark energy is the dominant force in our Universe. So long as stable atomic nuclei and the fabric of space do not undergo unexpected decays, and dark energy acts identically to the cosmological constant that it appears to be, this fate is inevitable.
The final era of dark energy dominance has already begun. Dark energy began to dominate the Universe’s energy content about the time our Sun and Solar System were born, and it became vital for the Universe’s expansion 6 billion years ago. The Universe may contain six distinct stages, but we’ve already been in the final one for the entirety of Earth’s history. Take a long look around at the Universe. It will never be this rich — or so easy to obtain — again.