Key Takeaways:

  1. The Universe is constantly evolving, with six distinct eras marking its history.
  2. We’re presently in the final era, dominated by dark energy, which began around 6 billion years ago.
  3. Each era is defined by significant shifts in energy density, temperature, and cosmic structure.
  4. The Universe started with an explosive expansion during the Inflationary era, preceding the hot Big Bang.
  5. The Dark Energy era will culminate in a desolate cosmos, with only isolated remnants enduring.

The Universe, an ever-changing tapestry of cosmic phenomena, undergoes subtle but profound transformations with each passing moment. These alterations, although imperceptible on human timescales, accumulate over cosmic epochs, reshaping the fabric of reality.

dark energy
While matter (both normal and dark) and radiation become less dense as the Universe expands owing to its increasing volume, dark energy, and also the field energy during inflation, is a form of energy inherent to space itself. As new space gets created in the expanding Universe, the dark energy density remains constant. (Credit: E. Siegel/Beyond the Galaxy)

The driving force behind this metamorphosis is the Universe’s expansion, causing the vast distances between cosmic structures to steadily increase.

In this grand cosmic ballet, everything possesses a certain quantum of energy – matter, radiation, dark energy, and more. As the Universe expands, the volume these energies occupy fluctuates, leading to distinct evolutions in their energy density. Matter’s density evolves as 1/a³, radiation’s as 1/a⁴, and dark energy remains constant (1/a⁰). Consequently, older Universes have expanded more, appearing cooler, larger, and gravitationally distinct from their earlier states.

A visual history of the expanding Universe includes the hot, dense state known as the Big Bang and the growth and formation of structure subsequently. The full suite of data, including the observations of the light elements and the cosmic microwave background, leaves only the Big Bang as a valid explanation for all we see. As the Universe expands, it also cools, enabling ions, neutral atoms, and eventually molecules, gas clouds, stars, and finally galaxies to form. (Credit: NASA/CXC/M. Weiss)
Our entire cosmic history is theoretically well-understood, but only because we understand the theory of gravitation that underlies it, and because we know the Universe’s present expansion rate and energy composition. Light will always continue to propagate through this expanding Universe, and we will continue to receive that light arbitrarily far into the future, but it will be limited in time as far as what reaches us. We will need to probe to fainter brightnesses and longer wavelengths to continue to see the objects presently visible, but those are technological, not physical, limitations. (Credit: Nicole Rager Fuller/National Science Foundation)

By applying the laws of physics, scientists can trace the Universe’s journey from its inception in the hot Big Bang to the enigmatic era of cosmic inflation. These extrapolations also offer a glimpse into the distant future, revealing the ultimate fate that awaits all existence.

Inflationary Era:

At the high temperatures achieved in the very young Universe, not only can particles and photons be spontaneously created, given enough energy, but also antiparticles and unstable particles as well, resulting in a primordial particle-and-antiparticle soup. Yet even with these conditions, only a few specific states, or particles, can emerge. (Credit: Brookhaven National Laboratory)

Before the hot Big Bang, the Universe pulsed with a unique energy inherent to space itself, propelling an exponential expansion.

This rapid growth rendered the Universe spatially flat, vastly exceeding its visible horizon, and eradicating any lingering particles. The quantum fluctuations of this era sowed the seeds of the cosmic structure we witness today, marking the dramatic transition from inflation to the hot Big Bang.

Primordial Soup Era:

Following the hot Big Bang, the Universe, now filled with matter, antimatter, and radiation, embarked on a cooling journey.

At early times (left), photons scatter off of electrons and are high-enough in energy to knock any atoms back into an ionized state. Once the Universe cools enough, and is devoid of such high-energy photons (right), they cannot interact with the neutral atoms, and instead simply free-stream, since they have the wrong wavelength to excite these atoms to a higher energy level. (Credit: E. Siegel/Beyond the Galaxy)

Particle collisions generated particle-antiparticle pairs, governed by the iconic equation E=mc². Within seconds, antimatter vanished, leaving behind only matter. After several minutes, stable deuterium formed, and nucleosynthesis of light elements commenced.

Plasma Era: In this phase, charged particles, outnumbered immensely by photons, grappled with continuous disruption. The Universe’s energy was primarily carried by radiation initially, but ultimately shifted towards normal and dark matter. The end of this era, 380,000 years post-Big Bang, ushered in the dominance of neutral matter.

Schematic diagram of the Universe’s history, highlighting reionization. Before stars or galaxies formed, the Universe was full of light-blocking, neutral atoms. While most of the Universe doesn’t become reionized until 550 million years afterwards, with some regions achieving full reionization earlier and others later. The first major waves of reionization begin happening at around 250 million years of age, while a few fortunate stars may form just 50-to-100 million years after the Big Bang. With the right tools, like the James Webb Space Telescope, we may begin to reveal the earliest galaxies. (Credit: S. G. Djorgovski et al., Caltech. Produced with the help of the Caltech Digital Media Center)

Dark Ages Era: Filled with neutral atoms, this era saw the commencement of structure formation through gravitational interactions. However, visible light was obstructed by cosmic dust, rendering the Universe seemingly dark. Reionization of the intergalactic medium, fueled by intensive star-formation, marked the end of this epoch, illuminating the cosmic expanse.

The galaxy cluster Abell 370, shown here, was one of the six massive galaxy clusters imaged in the Hubble Frontier Fields program. Since other great observatories were also used to image this region of sky, thousands of ultra-distant galaxies were revealed. By observing them again with a new scientific goal, Hubble’s BUFFALO (Beyond Ultra-deep Frontier Fields And Legacy Observations) program will obtain distances to these galaxies, enabling us to better understand how galaxies formed, evolved, and grew up in our Universe. When combined with intracluster light measurements, we could gain an even greater understanding, via multiple lines of evidence of the same structure, of the dark matter inside. (Credit: NASA, ESA, A. Koekemoer (STScI), M. Jauzac (Durham University), C. Steinhardt (Niels Bohr Institute), and the BUFFALO team)

Stellar Era: With the dark ages receding, stars and galaxies became observable, embedded within a growing cosmic web. Dark and normal matter dominated the energy landscape, fostering the birth and evolution of galaxies. Over time, however, the star formation rate dwindled, leading to a shift in energy dominance towards dark energy.

dark energy
The different possible fates of the Universe, with our actual, accelerating fate shown at the right. After enough time goes by, the acceleration will leave every bound galactic or supergalactic structure completely isolated in the Universe, as all the other structures accelerate irrevocably away. We can only look to the past to infer dark energy’s presence and properties, which require at least one constant, but its implications are larger for the future. (Credit: NASA & ESA)

Dark Energy Age: The current and final era witnesses a peculiar turn of events. Large-scale structures cease to expand, leaving unbound objects adrift in an ever-expanding cosmos. Over time, galaxies will merge into one massive elliptical galaxy, stars will fade, and new formations will cease. Ultimately, only isolated remnants will persist, held together by dark energy.

After the sun becomes a black dwarf, if nothing ejects or collides with the remnants of Earth, eventually gravitational radiation will cause us to spiral in, be torn apart, and eventually swallowed by the remnant of our sun. (Credit: Jeff Bryant/Vistapro)

In this dark energy-dominated age, the fate of the Universe is sealed, with only black dwarf stars and minute masses remaining. This era began around 6 billion years ago, and for the entirety of Earth’s history, we’ve been in this final act. The Universe, as we know it, will never again be as accessible or rich in cosmic wonders.

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