What do we know about dark matter and dark energy in our Universe?

What do we know about dark matter and dark energy in our Universe?

Dark matter and energy are our Universe's most mysterious and fascinating components. Despite their enigmatic nature, scientists have made significant progress in understanding these elusive substances and their role in the cosmos.

Let's start by discussing dark matter, a hypothetical form of matter that does not emit, absorb, or reflect light, making it invisible and undetectable by conventional means. The existence of dark matter was first proposed by Swiss astronomer Fritz Zwicky in the 1930s, who noticed discrepancies in the rotational speeds of galaxies that could not be explained by visible matter alone. Since then, numerous lines of evidence from various astrophysical observations have supported the existence of dark matter.

One of the most compelling evidence for dark matter is the study of galaxy clusters. By observing the gravitational effects of these massive structures on the light emitted by distant galaxies, astronomers have been able to infer the presence of vast amounts of unseen mass. The most widely accepted explanation for this additional mass is that it consists of dark matter, which interacts gravitationally with ordinary matter but does not emit any detectable radiation.

Another key evidence for dark matter comes from studies of the cosmic microwave background (CMB) radiation, which is the afterglow of the Big Bang. By analyzing the patterns of temperature fluctuations in the CMB, scientists have been able to infer the distribution of matter in the early Universe. These observations suggest that dark matter is crucial in forming cosmic structures such as galaxies and clusters.

Despite the overwhelming evidence for the existence of dark matter, its true nature remains a mystery. Various theoretical models have been proposed to explain dark matter, with the most popular candidate being a type of particle that interacts weakly with ordinary matter, known as a weakly interacting massive particle (WIMP). Efforts to detect these hypothetical particles directly have yet to be conclusive, leading to ongoing debates within the scientific community about the nature of dark matter.

Moving on to dark energy, this mysterious force was first proposed in the late 1990s to explain the unexpected accelerated expansion of the Universe. Before this discovery, it was widely believed that the Universe's expansion was slowing down due to the gravitational attraction of matter. However, observations of distant supernovae revealed that the Universe's expansion was speeding up, indicating the presence of a repulsive force known as dark energy.

The most common explanation for dark energy is the cosmological constant, a term introduced by Albert Einstein in his theory of general relativity. According to this theory, dark energy is a constant energy density that permeates all of space and exerts a negative pressure, causing the expansion of the Universe to accelerate. While the cosmological constant provides a simple and elegant explanation for dark energy, its value remains a central puzzle in cosmology, as it is many orders of magnitude smaller than theoretical predictions.

In addition to the cosmological constant, alternative explanations for dark energy have been proposed, such as quintessence, a dynamic form of dark energy that evolves. Quintessence models suggest that the nature of dark energy may change as the Universe expands, leading to different predictions for the future evolution of the cosmos.

One of the key challenges in studying dark energy is the need for direct observational evidence for its existence. Unlike dark matter, which exerts a gravitational influence on visible matter, dark energy is thought to be uniformly distributed throughout the Universe. It does not interact with ordinary matter or radiation. This makes it extremely difficult to detect and study dark energy directly, leading to ongoing efforts to understand its properties through indirect observations and theoretical models.

Despite the many uncertainties surrounding dark matter and energy, these enigmatic substances play a crucial role in shaping the structure and evolution of the Universe. From galaxies' formation to the cosmos' accelerating expansion, dark matter and energy represent some of the most profound mysteries in modern astrophysics. As scientists continue to probe the nature of these elusive components, our understanding of the Universe and its fundamental properties will surely deepen, unlocking new insights into the nature of reality itself.

A simulation of the dark matter distribution in the Universe 13.6 billion years ago.

Dark Matter and Dark Energy

Only five percent of the Universe is visible. What is the rest made up of?

The visible Universe—including Earth, the sun, other stars, and galaxies—comprises protons, neutrons, and electrons bundled together into atoms. Perhaps one of the most surprising discoveries of the 20th century was that this ordinary, or baryonic, matter makes up less than 5 percent of the Universe's mass.

The rest of the Universe appears to be made of a mysterious, invisible substance called dark matter (25 percent) and a force that repels gravity known as dark energy (70 percent).

Unlocking the Mystery

Scientists have yet to observe dark matter directly. It doesn't interact with baryonic matter and is entirely invisible to light and other electromagnetic radiation, making dark matter impossible to detect with current instruments. However, scientists are confident it exists because of the gravitational effects it appears to have on galaxies and galaxy clusters.

For instance, according to standard physics, stars at the edges of a spinning, spiral galaxy should travel much slower than those near the galactic center, where a sensible matter is concentrated. However, observations show that stars orbit at more or less the same speed regardless of where they are in the galactic disk. This puzzling result makes sense if one assumes that the boundary stars feel the gravitational effects of an unseen mass—dark matter—in a halo around the galaxy.

Dark matter could also explain certain optical illusions that astronomers see in the deep Universe. For example, pictures of galaxies that include strange rings and arcs of light could be explained if the light from even more distant galaxies is being distorted and magnified by massive, invisible clouds of dark matter in the foreground phenomenon known as gravitational lensing.

Scientists have a few ideas for what dark matter might be. One leading hypothesis is that dark matter consists of exotic particles that do interact with ordinary matter or light but exert a gravitational pull. Several scientific groups, including one at CERCERN's srge Hadron Collider, are currently working to generate dark matter particles for study in the lab.

Other scientists think the effects of dark matter could be explained by fundamentally modifying our theories of gravity. According to such ideas, there are multiple forms of gravity, and the large-scale gravity governing galaxies differs from the gravity to which we are accustomed.

Expanding Universe

Dark energy is even more mysterious, and its discovery in the 1990s shocked scientists. Previously, physicists had assumed that the attractive force of gravity would slow down the UniUniverse'spansion over time. However, when two independent teams tried to measure the deceleration rate, they found that the expansion was speeding up. One scientist likened the finding to throwing a set of keys up in the air, expecting them to fall back down only to see them fly straight up toward the ceiling.

Scientists now think the UniUniverse's Celebrated expansion is driven by a repulsive force generated by quantum fluctuations in otherwise "em" ty" s" ace. Moreover, the force seems to grow stronger as the Universe expands. Without a better name, scientists call this mysterious force dark energy.

Unlike for dark matter, scientists have no plausible explanation for dark energy. According to one idea, dark energy is a fifth and previously unknown type of fundamental force called quintessence, which fills the Universe like a fluid.

Many scientists have also pointed out that the known properties of dark energy are consistent with a cosmological constant, a mathematical Band-Aid that Albert Einstein added to his theory of general relativity to make his equations fit with the notion of a static universe. According to Einstein, the constant would be a repulsive force that counteracts gravity, keeping the Universe from collapsing. Einstein later discarded the idea when astronomical observations revealed that the Universe was expanding, calling the cosmological constant his "bi" gest blunder."

N"w that we see the UniUniverse'spansion accelerating, adding dark energy as a cosmological constant could neatly explain how space and time are being stretched apart. But that explanation still leaves scientists clueless about why the strange force exists in the first place's dark energy-matter content. Diagram showing the proportions of the Universe made up of dark energy (70%) and matter (30%). The matter can be dark (29%) or visible (1%). The visible matter (thUniverse's luminous stars) cannot explain thUniverse's observed expansion rate. This missing mass can be accounted for by MACHOs (massive astrophysical compact halo objects) such as brown dwarf stars or elementary particles like WIMPS (weakly-interacting massive particles). MACHOs are baryonic matter (containing protons and neutrons), and WIMPS are non-baryonic. Dark energy is a relatively unknown quantity driving the universe's expansion.

Dark Energy Survey census of the tiniest galaxies hones the search for dark matter

Today,. Scientists in the Dark Energy Survey released results that have been five years in the making. Researchers used the world’s most complete census of dwarf galaxies around our Milky Way galaxy to probe the nature of dark matter, an invisible form of matter that dominates the Universe. These new measurements provide information about what dark matter can and cannot be made of.

In particular, the new results constrain the minimum mass of the dark matter particles and the strength of interactions between dark matter and ordinary matter.

According to these new results, a dark matter particle must be heavier than a zeptoelectronvolt of 10-21 electronvolts. That’s one trillionth of a trillionth of the mass of an electron. This study also shows that dark matter’s interactions with ordinary matter must be roughly 1,000 times weaker than the weak nuclear force. Of the known forces, only gravity is weaker.

This shows the result of two numerical simulations predicting the distribution of dark matter around a galaxy similar to our Milky Way. The left panel assumes that dark matter particles were moving fast in the early Universe (warm dark matter), while the right panel assumes that dark matter particles were moving slowly (cold dark matter). The warm dark matter model predicts fewer small clumps of dark matter surrounding our galaxy and, thus, many fewer satellite galaxies that inhabit these tiny clumps of dark matter. By measuring the number of satellite galaxies, scientists can distinguish between these models of dark matter. Image: Bullock and Boylan-Kolchin (2017); simulations by V. Robles, T. Kelley, and B. Bozek, in collaboration with Bullock and Boylan-Kolchin

These novel measurements used data from the Dark Energy Survey, a cosmological survey designed to study dark energy, the mysterious force driving the universe's accelerated expansion. In contrast, dark matter is attractive, resisting the expansion of the Universe and gravitationally binding astronomical systems such as galaxies. The most miniature "e “dw" rf” galaxies can have hundreds to thousands of times more dark matter than ordinary matter. Over the past five years, the Dark Energy Survey has combined with other surveys to more than double the known population of these tiny galaxies. The current total is now over 5".

“The Dark Energy Survey analysis has added a stringent new test to the standard cosmological paradigm and places tight constraints on several alternative dark matter modes"s,” said Dark Energy Survey spokesperson Rich Kron, a University of Chicago and Fermilab scientist." “This is a fantastic example of how cosmological observations of the very large can inform particle physics experiments studying the very sma"l.”

Dark matter makes up 85% of the matter in the Universe, but we have yet to detect it directly in the laboratory. The gravitational effects of dark matter are indirectly visible in the motions of stars in galaxies, the clumpy distribution of galaxies in the Universe, and even in the amount of lightweight elements. The robust astronomical evidence for the existence of dark matter has motivated many experimental searches here on Earth, using instruments ranging from cryogenic detectors buried deep underground to energetic particle colliders".

“The faintest galaxies are among the most valuable tools we have to learn about dark matter because they are sensitive to several of its fundamental properties all at one "e,” said Ethan Nadler, study’s lead author and graduate student at Stanford University and SLAC.

By combining the observed census of dwarf galaxies with advanced cosmological simulations of the distribution of dark matter around the Milky Way, scientists could predict how the physical properties of dark matter would affect the number of small galaxies. Small galaxies form in regions where the dark matter density in the early Universe is very slightly above average. Physical processes that smooth out these regions of higher density (if dark matter moves too quickly or gains energy due to interactions with ordinary matter) or prevent density variations from collapsing to form galaxies (thanks to quantum interference effects) would reduce the number of galaxies observed by the Dark Energy Sur" It's“It’s exciting to see the dark matter problem attacked from so many different experimental an"les,” said Fermilab and University of Chicago scientist Alex Drlica-Wagner, Dark Energy Survey collaborator and one of the lead authors on the pa" er. “This is a milestone measurement for DES, and I’m very hopeful that future cosmological surveys will help us understand what dark matte" is.”

The Office of Science of the U.S. Department of Energy supports Fermilab. The Office of Science is the most prominent supporter of basic research in the physical sciences in the United States and is working to address some of the most pressing challenges of our time. For more information, visit science.energy.gov.

FROM SLAC, Tagged: California, cosmology, Dark Energy Survey, dark matter, SLAC

13/4/2024