Key Takeaways:
- Calculating the total mass of the universe is complex due to the prevalence of invisible matter, specifically dark matter (27%) and dark energy (68%), leaving only 5% as normal matter.
- Normal matter's mass is primarily estimated from measurements of cosmic microwave background radiation, while dark matter's mass is inferred from galactic rotation curves and gravitational lensing effects on light.
- Independent methods for mass estimation include analyzing the influence of matter (both normal and dark) on the universe's expansion rate since the Big Bang.
- Current estimates suggest the observable universe contains approximately 1023 stars, with a total mass of 1053 kilograms and 1080 atoms, predominantly attributable to dark matter and dark energy.
How is the total mass of the universe calculated?
Bob Giddings
Green Bay, Wisconsin
Calculating the total mass of the universe is not simple, because most of the mass is invisible.
In a pie chart of the contents of the universe, only 5 percent is normal matter, atoms that make up all the planets, stars, and galaxies. About 95 percent is composed of two invisible, enigmatic entities. Dark matter represents 27 percent of the universe, and dark energy is dominant, about 68 percent of the universe. Setting aside dark energy, these ingredients are measured in different ways.
The normal matter is almost entirely hydrogen and helium, the two simplest elements. All the other elements in the periodic table are just 2 percent of the census. And even the normal matter is mostly dark: All the stars in a trillion galaxies comprise just 0.5 percent of the contents of the universe. The bulk of normal matter is either too cold or too hot to be easily seen. Various methods have been used to locate normal matter, but the most accurate estimate comes from measurements of the cosmic microwave background radiation, the afterglow of the Big Bang.
Dark matter outweighs normal matter by a factor of five or six. It was first noticed 50 years ago in the way spiral galaxies rotate. They rotate too quickly to be held together by all their stars, so there must be an extended halo of invisible mass surrounding the visible stars and gas. The way stars move in a galaxy can be used to measure the total mass, both visible and dark. Astronomers don’t know what dark matter is, but suspect it is a new type of fundamental particle that doesn’t interact with radiation.
An independent way of measuring mass in the universe is gravitational lensing. General relativity, our best theory of gravity, predicts that mass bends light. In gravitational lensing, light from a distant object is deflected by an intervening galaxy or cluster of galaxies. By studying lensing, astronomers can estimate the mass of the intervening objects, including dark matter. This method agrees with estimates based on the motions of stars and galaxies.
The mass of the entire universe can also be estimated by measuring how the expansion rate has changed since the Big Bang. Normal matter and dark matter have acted to decelerate cosmic expansion, while in the past few billion years, dark energy has dominated and is accelerating the expansion rate. Matching the changing expansion rate to a Big Bang model gives the mass of the universe. The mass is insufficient to overcome the effects of dark energy, so the universe will expand forever.
Estimating the mass of the universe leads to some spectacular numbers. There are about a trillion galaxies in the observable universe. All the normal matter in those galaxies comprises 1023 stars, 1053 kilograms, and 1080 atoms.
Chris Impey
Distinguished Professor of Astronomy, Department of Astronomy, University of Arizona, Tucson
