Human understanding of the history of the universe has become much more complicated lately. The observable skies are a little cluttered — by something we can’t see. The mysterious force of dark energy, coupled with the unknown quality of dark matter, has left the brightest minds in astrophysics scratching their heads
This article is an introduction to dark energy and dark matter, and their implications for our understanding of the universe. As is typical with any study of the universe, we’ll need to look back in time, but we’ll stick — more or less — to a past-to-present-day timeline
Why Is Dark Energy Important to Understanding the Universe?
We need to understand the origins of the universe before we look at dark energy and dark matter in detail. Much written about dark energy and dark matter is theoretical and unproven, with new ways of approaching the origin of the universe taking root. However, this brief introduction will help us understand the latest ideas about dark energy and dark matter
The Big Bang Theory describes how the universe started as a hot, dense, radioactive state. The universe expanded rapidly, cooling down and becoming less dense. During this rapid expansion, matter formed — matter being something that takes up space by having volume. Objects with mass can be weighed
Radiation was also present, and radiation is essential to cosmologists in their work. The cosmic microwave background, or CMB, is radiation left over from the Big Bang. CMB has a uniform temperature. Cosmologists seek changes in CMB temperatures to help unravel the mysteries of the universe
How Do We Know the Universe Is Expanding?
The Big Bang theory is well known to 21st-century citizens. However, American cosmologist and astronomer Edwin Hubble only discovered the expanding universe less than a century ago.
It was in 1929 that Hubble published his world-changing PNAS paper, which discussed the observable relationship between distance and the recession speed traveled by galaxies. Hubble had observed many different clusters of galaxies moving away from Earth. He devised a calculation that showed the more-distant galaxies recede faster than those closer to Earth. In other words, Hubble discovered the expanding universe. How Hubble did this will help us later with understanding dark matter and dark energy
As light travels to Earth from distant galaxies, its light waves stretch. As space is getting bigger and expanding, light waves stretch over this ever-expanding space. The stretching makes wavelengths get longer, and longer visible wavelengths look redder to the human eye. This is called redshift.
Hubble studied many galaxies deep in the universe. The changing red light waves helped Hubble reach his conclusion about space and its expansion
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How Do We Know How Fast the Universe Is Expanding?
Some 15 years before Hubble’s discovery, Albert Einstein had published his theory of general relativity. Einstein’s theory holds that gravity remains an attractive force for all forms of energy and matter, whether on Earth or a cosmic scale
Gravity, according to the theory, would eventually bring deceleration to the expansion of the universe. We’ll come back to Einstein’s theory of general relativity shortly. First, we’re now ready to talk about dark matter
How Was Dark Matter Discovered?
Evidence of dark matter first came to the attention of Swiss-American astronomer Fritz Zwicky in the 1930s. Zwicky was studying galaxies’ movements within a cluster called the Coma Cluster. Based on their visible matter, his calculations of their expected speed were wrong; the galaxies were traveling faster than expected
Particle physics suggested the existence of undiscovered particles in the 1960s. Could there be a link?
American astronomer Vera Rubin saw something similar to Zwicky in the 1970s. Rubin noted that the outer regions of spiral galaxies were rotating faster than expected. Both Rubin and Zwicky had observed something adding to the force of gravity impacting these galaxies
American physicist James Peebles took the studies further in 1974. Previous models for measuring galaxies’ mass looked at their visible matter. However, using galaxies’ movements to assess their mass found galaxies to be much heavier.
The imbalance of matter and antimatter adds to the confusion around dark matter. Early universe theories posit that matter and antimatter should exist in equal quantities. Yet, there’s a lot more matter in the universe. Does dark matter have a hand in this?
This “missing matter” gave birth to the existence of dark matter; something was providing more mass, and therefore more matter. The unseen matter was given the title “dark matter.”
New calculation methods, which took dark matter into account, now balanced when measuring the mass of galaxies and large-scale structures such as clusters of galaxies
Is Something Else Other Than Gravity Slowing the Universe?
In 1998, NASA’s Hubble Space Telescope dropped a scientific Big Bang of its own. Astronomers had been training the powerful Hubble telescope onto very distant supernovae, a supernova being the explosion of a star
Type Ia supernovae are very bright in the sky. The Hubble telescope supernova studies showed that, a long time ago, the universe had a slower expansion rate than today. Previous work assumed the expansion of the universe was accelerating.
The previous accelerated-expansion theory was shot, and no one had any answers initially. Experts looked for imperfections in Einstein’s theory, and questioned gravity and matter, to no avail
Introduction to Dark Matter and Dark Energy
Our cosmic journey has finally brought us to dark matter and dark energy’s door. These menacing names make them feel like villains in a comic book story. Very little is understood about how either of these interacts with the universe. Let’s knock and see what’s behind this cosmic gateway
Who Discovered Dark Energy and Dark Matter?
In 2011, three astrophysicists won the 2011 Nobel Prize in Physics for their work on dark energy and dark matter
Saul Perlmutter, Brian Schmidt, and Adam Riess were honored “for the discovery of the accelerating expansion of the universe through observations of distant supernovae.”
Perlmutter had launched the Supernova Cosmology Project in 1988 to measure the universe’s expansion. In 1994, Perlmutter was joined by The High-z Supernova Search Team led by Schmidt and in which Riess was a vital cog
They anticipated recording the gradual slowing down of the universe because of gravity. What they found was that dark energy was a repulsive force. Dark energy was the universe’s dominant component, and it was accelerating the expansion of the universe. But still no one could explain what dark energy or dark matter was
How Much of the Universe Is Dark Matter and Dark Energy?
In 2013, a European-led research team from the European Space Agency (ESA) published its dark energy survey. A high-precision probe called Planck had mapped the sky’s cosmic microwave background (CMB), looking for missing mass
Planck found that just 5% of the universe’s total mass was normal matter, also known as Baryonic matter. Baryonic matter includes the protons, neutrons, and electrons that make planets, stars, brown dwarfs, and more — anything that has mass and can be weighed.
Planck measured Baryon acoustic oscillations (BAO) in the cosmic microwave background (CMB), looking for fluctuations that helped cosmologists decipher the early universe. Surprisingly, some 27% of the universe was dark matter, and 68% of the matter-energy density is dark energy
What Is Dark Matter?
Before we look at dark matter candidates, we need to look briefly at the Standard Model of Particle Physics.
This standard model describes the universe’s building blocks of all known matter. The model includes electrons, photons, and the famous Higgs boson, the smallest particle in the standard model. The Higgs boson was observed in 2012 by scientists using the Large Hadron Collider (LHC) for the European Council for Nuclear Research (CERN)
Ordinary matter is baryonic matter. Dark matter particles are also called unknown non-baryonic matter. Neither dark matter nor dark energy currently forms part of the Standard Model of Particle Physics
Theories for the properties of dark matter include:
- Weakly interacting massive particles, or WIMPs, which do not emit or absorb light and, as the name suggests, interact weakly with other particles. However, they can bang into each other, and when they do, they annihilate and produce gamma-rays.
- Axions are hypothetical particles. They’re subatomic particles, believed to be cold and slow-moving particles produced in the early universe.
- Neutrinos are ghostly, mysterious particles, of which dark matter could be a standard neutrino, a massive neutrino, or even a sterile neutrino.
- Neutralinos, large but light particles, but undetected and only theorized thus far.
NASA’s Fermi Large Area Telescope (LAT) scans the Milky Way and beyond, looking for gamma-rays that may give us clues about dark matter. Gamma-rays are an energy-intense radiation form, a form of electromagnetic radiation that emits radiation from black holes, supernovas, and more.
How Does Dark Matter Interact With the Universe?
The name dark matter is slightly confusing because it suggests something dark or black, perhaps how many of us visualize space in our solar system and beyond. Dark matter is invisible but has mass and therefore gravitational effects in space, but we don’t know why it has weight
Dark matter doesn’t absorb light or stop light from passing through it. It doesn’t interact with anything we know. But we do know it interacts with gravity because it distorts light. How?
Cosmologists look at stars, galaxies, and supernovae many light-years away from Earth. Dark matter distorts the light that shines from these objects and travels to Earth. This distortion effect is called gravitational lensing
Remember Einstein? Albert Einstein took Newton’s take on gravity to a new level. Newton didn’t think objects with mass could affect space; Einstein believed mass could pull, bend, and warp space
Einstein’s theory of relativity linked space, gravity, and time together into a single continuum known as space-time. Einstein took the three dimensions of space (up and down, left and right, back and forth) and linked them to time to create a fourth dimension.
Einstein realized that objects with mass, like planets, distort space-time. Entities with large masses can slow or speed up time, and the stronger the gravity, the slower time proceeds.
For example, GPS satellites orbit the Earth at around 14,000 kilometers (8,700 miles) per hour. They carry clocks, and these clocks are traveling a lot faster than clocks on Earth. Amazingly, the clocks go out of sync by 38 microseconds per day
What Is the Definition of Dark Energy?
Our cosmic ride has now led us back to our beginning, which is dark energy’s doorstep
Dark energy, like dark matter, is another unidentified part of the universe’s make-up. But as 68% of the known universe, we need to understand the mystery around this form of energy
Much of space is a vacuum. Dark energy can be thought of as vacuum energy, meaning where there’s a vacuum in space, there’s dark energy. As we know, the universe is expanding — creating more space, which creates more vacuum. The more vacuum in space there is, the more dark energy is present. In turn, the more dark energy that exists, the more influence dark energy can have
Hypothetically, dark energy is a repulsive form of energy that exerts negative pressure on large-scale structures. In other words, it behaves oppositely to gravity, making the universe expand more rapidly. Dark energy may have far-reaching implications for our understanding of the universe
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What Are the Current Theories About Dark Energy?
Some believe dark energy is simply an intrinsic part of the expansion of the universe, what’s called a cosmological constant. In other words, as space and the universe expand, more dark energy is created
Some cosmologists believe dark energy could be a previously unknown energy field or a fluid. This theory calls the substance that fills space quintessence, albeit there’s no actual proof of why quintessence exists, what it’s like, or how it interacts with the universe
Other ideas include dark energy’s ever-accelerating expansion of the universe leading to an apocalyptic scenario where holes tear through galaxies and planets. This destruction of the universe is called the Big Rip.
What if dark energy suddenly changes? Or if the pulling factor of gravity overcomes its battle with the repulsive properties of dark energy? Should gravity “win” and the universe compresses, everything would eventually be squashed together, resulting in an apocalyptic ending called the Big Crunch
Can Dark Energy Be Harnessed?
Harnessing dark energy is in the theoretical stage — we have yet to define it, see it, or understand it, so harnessing it is a ways off still
Can Dark Energy Be Destroyed?
We still don’t fully understand dark energy well enough to know how it’s created or if it can be destroyed. Dark energy may convert into baryonic matter, decay, or have several as-yet-unknown properties.
Dark Matter and Dark Energy Remain a Mystery
Much is unknown about dark matter and dark energy. Many of the greatest minds in the world are looking at the skies and using physics to help
