Exploring the Enigma of Dark Energy Within Our Expanding Universe

Approximately 13.8 billion years ago, the universe commenced with a rapid expansion known as the Big Bang. Following this initial burst, which lasted only a fraction of a second, gravity began to decelerate the universe. However, the cosmos did not remain in this state. Around nine billion years after its inception, the universe’s expansion started to accelerate, propelled by an unidentified force referred to as dark energy.

Nevertheless, the true nature of dark energy remains elusive.

In essence, we lack a definitive answer. Nonetheless, we are aware of its existence and its role in driving the universe to expand at an increasing pace. Dark energy accounts for approximately 68.3% to 70% of the entire universe.

It all started with Cepheids

The existence of dark energy was not known until the late 1990s, but its roots in scientific research can be traced back to 1912. It was during this time that American astronomer Henrietta Swan Leavitt made a significant breakthrough by studying Cepheid variables, a specific type of stars that experience fluctuations in brightness with a regular pattern based on their own brightness.

Leavitt discovered that all Cepheid stars with a specific period (the time it takes for them to go from bright, to dim, and back to bright again) have the same absolute magnitude or luminosity, which refers to the amount of light they emit. By measuring these stars, Leavitt established a correlation between their regular brightness period and luminosity. This breakthrough allowed astronomers to utilize the period and luminosity of Cepheid stars to calculate the distances between us and these stars in distant galaxies, including our own Milky Way.

During this same period, astronomer Vesto Slipher made observations of spiral galaxies using a spectrograph attached to his telescope. The spectrograph, a relatively new invention at the time, splits light into its constituent colors, similar to how a prism creates a rainbow. Slipher used this device to analyze the different wavelengths of light emitted by galaxies, known as spectral lines. Through his observations, Slipher became the first astronomer to notice the redshift phenomenon, which refers to the speed at which a galaxy is moving away from us. These observations would later prove crucial in various scientific breakthroughs, including the eventual discovery of dark energy.

Redshift is a term used to describe the phenomenon when astronomical objects move away from us, causing the light emitted by these objects to stretch out. Since light behaves like a wave, red light has the longest wavelength. Therefore, the light from objects moving away from us exhibits a longer wavelength, shifting towards the “red end” of the electromagnetic spectrum.

Discovering an expanding universe

The observation of galactic redshift, the period-luminosity relation of Cepheid variables, and the ability to measure the distance of stars and galaxies played a crucial role in astronomers’ realization that galaxies were moving farther away from us as time passed, indicating the expansion of the universe. Subsequently, scientists from various countries began piecing together the concept of an expanding universe.

In 1922, Alexander Friedmann, a Russian scientist and mathematician, published a paper that explored different possibilities regarding the history of the universe. This paper, based on Albert Einstein’s theory of general relativity from 1917, included the notion of an expanding universe.

In 1927, Georges Lemaître, a Belgian astronomer who was reportedly unaware of Friedmann’s work, also incorporated Einstein’s theory of general relativity into his own paper. Contrary to Einstein’s belief in a static universe, Lemaître demonstrated how the equations in Einstein’s theory actually supported the idea of an expanding universe.

In 1929, astronomer Edwin Hubble provided confirmation of the universe’s expansion through observations made by his colleague, Milton Humason. Humason measured the redshift of spiral galaxies, while Hubble and Humason studied Cepheid stars within those galaxies to determine their distances.

By comparing the distances of these galaxies with their redshift, Hubble and Humason discovered that the farther an object is, the greater its redshift and the faster it is moving away from us. This finding, now known as Hubble’s Law or the Hubble-Lemaître law, confirmed the reality of the expanding universe.

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The discovery of galactic redshift, the period-luminosity relation of Cepheid variables, and the ability to measure the distance of stars and galaxies played a significant role in astronomers’ understanding of the expanding universe. Over time, it became evident that galaxies were moving farther away from us, indicating the expansion of the universe. This realization led scientists from different parts of the world to piece together the concept of an expanding universe.

In 1922, Alexander Friedmann, a Russian scientist and mathematician, published a paper that explored various possibilities regarding the history of the universe. Building upon Albert Einstein’s theory of general relativity, which was published in 1917, Friedmann’s paper included the idea of an expanding universe.

In 1927, Georges Lemaître, a Belgian astronomer who was reportedly unaware of Friedmann’s work, also incorporated Einstein’s theory of general relativity into his own

Expansion is speeding up, supernovae show

Scientists had previously believed that the expansion of the universe would eventually be slowed down by the force of gravity, in accordance with Einstein’s theory of general relativity. However, in 1998, everything changed when two separate teams of astronomers studying distant supernovae noticed that these stellar explosions appeared dimmer than expected at a certain redshift. These teams, led by astronomers Adam Riess, Saul Perlmutter, and Brian Schmidt, were awarded the 2011 Nobel Prize in Physics for their groundbreaking work.

Although dim supernovae may not initially seem significant, these astronomers were specifically studying Type 1a supernovae, which are known to have a consistent level of brightness. Therefore, they deduced that there must be another factor causing these objects to appear dimmer. By measuring an object’s brightness, scientists can determine its distance and speed. Typically, dimmer objects are located farther away, although factors such as surrounding dust can also contribute to dimming.

Based on these observations, the scientists concluded that these supernovae were much farther away than initially anticipated based on their redshifts. By analyzing the brightness and spectrum of the objects, the researchers were able to calculate the distance and velocity at which these supernovae were moving away from us. They discovered that the supernovae were not as close as expected, indicating that they had traveled a greater distance away from us at a faster rate than previously thought. These findings ultimately led scientists to conclude that the universe itself is expanding at an accelerated pace over time.

While alternative explanations for these observations have been explored, astronomers studying even more distant supernovae and other cosmic phenomena in recent years have continued to gather evidence supporting the notion of cosmic acceleration – the idea that the universe is expanding at an increasingly rapid rate. However, as scientists have built a case for cosmic acceleration, they have also sought to understand the underlying cause driving this phenomenon.

What exactly is dark energy?

Currently, astronomers have labeled dark energy as the enigmatic force responsible for the accelerated expansion of the universe. Some have likened dark energy to a negative pressure that propels space outward, but it remains uncertain whether dark energy exerts any specific force. Numerous conjectures exist regarding the true nature of dark energy, and it is plausible that it may be something entirely different.

Vacuum energy

Some scientists propose that dark energy is a fundamental energy present throughout space known as vacuum energy. It is believed to be equivalent to the cosmological constant, a mathematical term in Einstein’s theory of general relativity. Initially, the constant was introduced to counteract gravity and maintain a static universe. However, when Hubble’s observations confirmed the expansion of the universe, Einstein discarded the constant, referring to it as his “biggest blunder,” as stated by physicist George Gamow.

Subsequently, when it was discovered that the universe’s expansion was accelerating, some scientists suggested the possibility of a non-zero value for the previously discredited cosmological constant. They proposed that an additional force, attributed to “vacuum energy,” was necessary to drive the expansion. This hypothetical component is believed to be a background energy that permeates all of space.

According to quantum field theory, space is never completely empty. It is postulated that virtual particles, consisting of particle-antiparticle pairs, continuously emerge and annihilate each other. This process is thought to be facilitated by the presence of “vacuum energy,” which fills the cosmos and exerts a pushing force on space.

While this theory has garnered significant attention, scientists investigating this concept have calculated the expected amount of vacuum energy in space. Their calculations indicate that either there should be an excessive amount of vacuum energy, causing the universe to expand so rapidly at its inception that stars and galaxies could not have formed, or there should be no vacuum energy at all. This implies that the actual amount of vacuum energy in the cosmos must be significantly smaller than predicted. However, this discrepancy remains unresolved and has been dubbed “the cosmological constant problem.”

Quintessence

Some scientists think that dark energy could be a type of energy fluid or field that fills space, behaves in an opposite way to normal matter, and can vary in its amount and distribution throughout both time and space. This hypothesized version of dark energy has been nicknamed quintessence after the theoretical fifth element discussed by ancient Greek philosophers.

It’s even been suggested by some scientists that quintessence could be some combination of dark energy and dark matter, though the two are currently considered completely separate from one another. While the two are both major mysteries to scientists, dark matter is thought to make up about 85% of all matter in the universe.

Space wrinkles

Some scientists think that dark energy could be a sort of defect in the fabric of the universe itself; defects like cosmic strings, which are hypothetical one-dimensional “wrinkles” thought to have formed in the early universe.

A flaw in general relativity

Some researchers argue that dark energy may not be a tangible entity that can be detected. Instead, they propose that the discrepancy lies within the framework of general relativity and Einstein’s theory of gravity, particularly when applied to the vast expanse of the observable universe. According to this perspective, scientists believe it is conceivable to revise our understanding of gravity in a manner that accounts for the observations of the universe without invoking the existence of dark energy. As early as 1919, Einstein himself put forth a concept known as unimodular gravity, which is a modified version of general relativity that contemporary scientists believe could provide a coherent explanation for the universe without the need for dark energy.

The future

The enigma of dark energy remains one of the most profound mysteries in the vast expanse of the universe. Over the course of several decades, scientists have tirelessly formulated theories to comprehend the expansion of our universe. Now, equipped with groundbreaking tools, we stand on the precipice of an unprecedented opportunity to put these theories to the ultimate test and delve into the fundamental question: “What exactly is dark energy?”

Within the realm of this cosmic exploration, NASA assumes a pivotal role in the Euclid mission, a collaborative effort with the European Space Agency (ESA) that was launched in 2023. Euclid aims to construct a three-dimensional map of the universe, enabling us to witness the gradual disintegration of matter under the influence of dark energy throughout time. This comprehensive map will encompass observations of billions of galaxies, spanning a distance of up to 10 billion light-years from our planet.

Furthermore, NASA’s forthcoming Nancy Grace Roman Space Telescope, scheduled for launch by May 2027, has been meticulously designed to investigate the enigmatic nature of dark energy, alongside numerous other scientific pursuits. This remarkable telescope will also generate a three-dimensional map of dark matter. With a resolution comparable to that of NASA’s esteemed Hubble Space Telescope, Roman will possess a field of view a hundred times larger, enabling it to capture more expansive and intricate images of the universe. This unprecedented capability will empower scientists to meticulously chart the intricate structure and distribution of matter across the cosmos, while unraveling the behavior and evolution of dark energy over time. Additionally, Roman will undertake an additional survey to detect Type Ia supernovae, further enriching our understanding of the universe.

In conjunction with NASA’s endeavors, the Vera C. Rubin Observatory, currently under construction in Chile and supported by a vast collaboration that includes the U.S. National Science Foundation, stands poised to contribute significantly to our burgeoning comprehension of dark energy. This ground-based observatory is anticipated to commence operations in 2025.

The collective efforts of the Euclid, Roman, and Rubin missions herald the dawn of a new “golden age” in the field of cosmology. Through these ambitious endeavors, scientists will amass an unprecedented wealth of detailed information, unraveling the profound enigmas surrounding dark energy and propelling our understanding of the universe to unprecedented heights.

Furthermore, the James Webb Space Telescope, launched by NASA in 2021, is the largest and most powerful space telescope in the world. Its primary objective is to contribute to various research areas and specifically aid in the study of dark energy.

In addition to the James Webb Space Telescope, NASA has another mission called SPHEREx (the Spectro-Photometer for the History of the Universe, Epoch of Reionization, and Ices Explorer) scheduled to launch by April 2025. The purpose of this mission is to explore the origins of the universe. By surveying the entire sky in near-infrared light, including a vast number of galaxies, SPHEREx aims to gather valuable data that could enhance our understanding of dark energy.

Moreover, NASA actively supports a citizen science initiative known as Dark Energy Explorers. This project allows individuals from all around the world, regardless of their scientific background, to contribute to the search for answers regarding dark energy.

Lastly, it is important to note that dark energy and dark matter are not the same. Although we have yet to fully comprehend their nature, the main similarity between them is that their true identities remain unknown.

This article is republished from PhysORG under a Creative Commons license. Read the original article.