Matter and Mass of the Observable Universe
The scale of the cosmos is nearly impossible to grasp using human intuition. From the staggering number of stars to the invisible forces shaping the void, the observable universe—the portion of the universe from which light has had time to reach us—is a vast expanse of matter and energy. By applying mathematical models and astronomical observations, scientists can estimate the total mass and the number of particles that constitute our cosmic neighborhood.
Key Facts
- The observable universe contains an estimated 2 trillion galaxies.
- Ordinary matter (baryonic matter) makes up only 4.8% of the total critical density.
- The estimated mass of ordinary matter in the observable universe is approximately 1.46 × 1053 kg.
- The total number of atoms is estimated at 1080, a value known as the Eddington number.
- Dark energy is the dominant component of the universe, accounting for 68.3% of its density.
The Scale of Galaxies and Stars
Estimates regarding the number of galaxies vary, but some calculations suggest as many as 2 trillion galaxies within the observable universe. Other estimates place this number in the hundreds of billions. Regardless of the specific count, the number of stars is even more profound, estimated at 1022 to 1024. This means there are potentially more stars—and Earth-like planets—than there are grains of sand on all the beaches of Earth.
These numbers may be even higher depending on the model of cosmic inflation (the theory of rapid exponential expansion in the early universe). If the universe expanded by more than 60 e-folds, the total number of stars could exceed 10100.
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Calculating the Mass of Ordinary Matter
When astronomers discuss the mass of the observable universe, they are typically referring to ordinary matter, also known as baryonic matter. This includes everything made of protons and neutrons, such as stars, planets, the interstellar medium (ISM), and the intergalactic medium (IGM). It specifically excludes dark matter and dark energy.
Critical Density and the Friedmann Equations
To determine this mass, scientists use the concept of critical density—the energy density required for the universe to be "flat." In a universe without dark energy, this density represents the tipping point between continued expansion and an eventual collapse. This is calculated using the Friedmann equations:
ρc = 3H2 / 8πG
In this formula, G represents the gravitational constant and H is the Hubble constant (the unit of measurement used to describe the expansion of the universe). According to data from the European Space Agency's Planck Telescope, H0 is 67.15 kilometres per second per megaparsec. This results in a critical density of 0.85 × 10-27 kg/m3, or roughly five hydrogen atoms per cubic metre.
Composition of the Universe
The total critical density is not composed of a single substance but is divided among four primary components:
- Dark Energy: 68.3%
- Cold Dark Matter: 26.8%
- Ordinary Matter: 4.8%
- Neutrinos: 0.1%
While neutrinos are Standard Model particles, they are categorized separately because they are ultra-relativistic, meaning they behave more like radiation than traditional matter.
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From Density to Total Mass and Atoms
To find the total mass of ordinary matter, the density (4.08 × 10-27 kg/m3) is multiplied by the volume of the observable universe. Given that the universe has expanded for 13.8 billion years, the comoving radius is approximately 46.6 billion light-years. This results in a volume of 3.58 × 1080 m3 and a total ordinary mass of 1.46 × 1053 kg.
The Eddington Number
By assuming the mass of ordinary matter consists primarily of hydrogen atoms (which make up about 74% of the mass of atoms in the Milky Way), scientists can estimate the total number of atoms in the observable universe. By dividing the total mass of ordinary matter by the mass of a single hydrogen atom, the result is approximately 1080 atoms, referred to as the Eddington number.
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Summary of Cosmic Composition
| Component | Percentage of Critical Density | Estimated Value/Mass |
|---|---|---|
| Dark Energy | 68.3% | N/A |
| Cold Dark Matter | 26.8% | N/A |
| Ordinary Matter | 4.8% | 1.46 × 1053 kg |
| Neutrinos | 0.1% | N/A |
| Total Atoms | N/A | ~1080 (Eddington Number) |
Frequently Asked Questions
What is the difference between ordinary matter and dark matter?
Ordinary matter, or baryonic matter, consists of protons, neutrons, and electrons that make up stars, planets, and gas. Dark matter does not interact with light and is detected only through its gravitational effects, making up a much larger portion of the universe's density.
What is the Eddington number?
The Eddington number is the estimated total number of protons (or hydrogen atoms) in the observable universe, calculated to be approximately 1080.
How is the volume of the observable universe determined?
The volume is calculated using the comoving distance (radius), which is currently about 46.6 billion light-years, based on the fact that the universe has been expanding for 13.8 billion years.
Why are neutrinos listed separately from ordinary matter?
Although they are particles within the Standard Model, neutrinos are ultra-relativistic, meaning they move at speeds close to the speed of light and behave more like radiation than matter.
What happens if the cosmic inflation model is correct?
If the universe expanded by more than 60 e-folds during inflation, the total number of stars in the entire universe (beyond just the observable portion) could be as high as 10100.