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What is the universe made of?

Matter and energy are the two basic components of the entire Universe. An enormous challenge for scientists is that most of the matter in the Universe is invisible and the source of most of the energy is not understood. How can we study the Universe if we can’t see most of it?

Our Work

95%
The percentage of matter and energy in the Universe that is currently unobservable

As our tools for observation grow more sophisticated, scientists at Center for Astrophysics | Harvard & Smithsonian will continue to be at the forefront of dark matter and dark energy research.

NASA’s Chandra X-ray Observatory and optical telescopes help map the distribution of dark matter in colliding galaxy clusters, like the Bullet Cluster. X-ray observations show a heated shock front where the gas from the clusters collided and slowed down, but gravitational lensing measurements show that dark matter was unaffected by the collision and separate from the normal matter.

It is theorized that when some dark matter particles collide, they annihilate and disappear in a flash of high-energy radiation. The Very Energetic Radiation Imaging Telescope Array System (VERITAS) in Arizona, which can detect gamma-ray radiation, is looking for the signature of dark matter annihilation.

The South Pole Telescope in Antarctica and Chandra are placing limits on dark energy by looking for its effects on galaxy cluster evolution throughout the history of the Universe. By comparing observations of galaxy clusters with experimental models, researchers are studying how dark energy competed with gravity throughout the history of the Universe.

Scientists at CfA have led the Baryon Oscillation Spectroscopic Survey (BOSS), analyzing millions of galaxies and charting their distribution in the Universe. The distribution has been shown to trace sound waves from the early Universe, like ripples in a pond, where some regions have higher numbers of galaxies, and others have less. Looking at these distributions, we can more accurately measure the distance to galaxies and map the effects of dark energy.

On the horizon, the Dark Energy Spectroscopic Instrument (DESI) will create a 3D map of the Universe, containing millions of galaxies out to 10 billion light years. This map will measure dark energy’s effect on the expansion of the Universe. And the Large Synoptic Survey Telescope (LSST) will observe billions of galaxies and discover unprecedented numbers of supernovae, constraining the properties of dark matter and dark energy.

Dark Matter and Dark Energy

Astronomer Fritz Zwicky was the first to notice the discrepancy between the amount of visible matter in a cluster of galaxies and the motions of the galaxies themselves. He suggested that there may be invisible matter, or what he called “dark matter”, interacting gravitationally with the visible matter. Later, astronomers noticed similar incongruities when observing nearby spiral galaxies. The outer edges of the galaxies rotated much faster than expected, suggesting “dark matter” existed throughout and extended beyond the visible galaxy.

Today, we can estimate the amount of dark matter in a galaxy based on how it causes light from a background source to bend. Using this “gravitational lensing” technique, we can measure the severity of that bend to get an idea of the galaxy’s mass. When the mass we calculate from the bend and the mass we can observe directly don’t agree, we know dark matter must be present.

Modern calculations say dark matter comprises about 27% of the Universe. We don’t yet know what it is, but we are searching for answers.

We have known that the Universe is expanding since the early 20th century. But recent observations of distant supernovae and other observations show that the Universe is not only expanding, but the expansion is accelerating. This astonishing discovery came as a complete surprise because the expansion of the Universe should slow down with time because of the gravitational attraction between galaxies and clusters of galaxies. The unseen repellant force required to explain this observation has been labelled “dark energy,” and current models say it makes up about 68% of the Universe.

That leaves only 5% of the Universe that is visible to us. 

 

Supernova 1994D in this image from NASA's Hubble Space Telescope might look like a star, but it's the explosion of a white dwarf that nearly outshone an entire galaxy. Such supernovas — known as type Ia — are extremely similar to each other, allowing astronomers to use them to measure the rate of the expansion of the universe.

Credit: NASA/ESA, The Hubble Key Project Team and The High-Z Supernova Search Team

 

What We Know and What We Think

While we can’t see dark matter, we know it’s there. And we can investigate some of dark matter’s properties using gravitational lensing. This technique measures the gravitational pull galaxies exert on light from more distant sources. The warping and magnification of this light gives us insight into the amount, density, and distribution of dark matter in any given lensing galaxy. Theoretically, the current best explanation we have for dark matter is the existence of WIMPs, or Weakly Interacting Massive Particles. These theoretical particles should have certain predictable behaviors, but directly observing them and their byproducts so far has proved elusive.

As for dark energy, Einstein had assumed the Universe was static, neither expanding nor collapsing. However, his Theory of General Relativity predicted that the Universe was not static, and so he added a “cosmological constant,” to oppose gravity. He later called it the “biggest blunder” of his life after Hubble demonstrated that the Universe was expanding.

The discovery that the expansion of the Universe is accelerating revived the idea of the cosmological constant. The simplest interpretation of this constant is that it represents the energy of empty space. This “vacuum energy” is constant throughout space and time.

Another interpretation is that dark energy might be an energy field that varies over time and space. Or, perhaps we do not fully understand gravity. For example, maybe it acts differently on enormous scales. Astronomers are currently testing modifications to General Relativity to see if they can explain the Universe’s accelerating expansion.