How many atoms are there in the visible universe?

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Every matter in the universe – no matter how big or small or small or large – is made up of atoms.

Each of these building blocks consists of a positively charged nucleus, made up of protons and neutrons, and negatively charged electrons orbiting around them. The number of protons, neutrons, and electrons an atom has determines which element it belongs to on the periodic table and affects how it interacts with other atoms around it. All you see around you is just a composition of different atoms that interact with each other in unique ways.

So, if everything is made of atoms, do we know how many atoms are in the universe?

Related Topics: Why does outer space look black?

To start off “small,” there are about 7 octillion, or 7 x 10^27 (7 followed by 27 zeros), atoms in the average human body, according to The Guardian. Given this huge amount of atoms in just one person, you would think that it would be impossible to determine the number of atoms in the entire universe. And you’d be right: since we have no idea how large the entire universe is, we can’t know how many atoms are inside.

However, it is possible to roughly calculate how many atoms there are in the visible universe – the part of the universe we can see and study – using some cosmological assumptions and a bit of math.

visible universe

The universe originated during the Big Bang 13.8 billion years ago. When the universe exploded from a single point of infinite mass and temperature, the universe began expanding outward and hasn’t stopped since.

Since the universe is 13.8 billion years old and the observable universe extends as far from us as light can travel in time since the birth of the universe, you might assume that the observable universe extends only 13.8 billion light-years in each direction. But because the universe is constantly expanding, this is not the case. When we observe a distant galaxy or star, what we really see is where it was when the light was first emitted. But by the time the light reaches us, the galaxy or star is much farther away than it was when we saw it. Using the cosmic microwave background radiation, we can work out how fast the universe is expanding, and because this rate is constant – currently scientists’ best guess (although some scientists think it may be slowing) – it means that the observable universe actually spans 46 billion light-years across trends, according to Live Science’s sister site Space.com.

But knowing the size of the observable universe doesn’t tell us everything we know about the number of atoms in it. We also need to know how much substance or things are in it.

Note how the universe has expanded since the Big Bang 13.8 billion years ago. (Image credit: Shutterstock) Cosmic assumptions

But matter is not the only thing in the universe. In fact, they make up only about 5% of the universe, according to NASA. The rest is made up of dark energy and dark matter, but since it’s not made of atoms, you don’t have to worry about it because of this mystery.

Related: What’s Happening in Intergalactic Space?

According to Einstein’s famous equation E = mc^2, energy and mass, or matter, are interchangeable, so it is possible to create matter or convert it into energy. But on the cosmological scale of the universe, we can assume that the amount of formed and uncreated matter cancels each other out. This means matter is finite, so there are the same number of atoms in the observable universe as always, according to Scientific American. This is important because our image of the observable universe is not a single snapshot in time.

According to our observations of the known universe, the physical laws that govern it are the same everywhere. Combined with the assumption that the expansion of the universe is constant, this means that on a large scale, matter is uniformly distributed throughout the universe – a concept known as the cosmological principle. In other words, there are no regions in the universe that contain more matter than others. This idea allows scientists to accurately estimate the number of stars and galaxies in the observable universe, which is useful because most atoms are found inside stars.

simplify the equation

Knowing the size of the observable universe and that matter is evenly and finitely distributed across it makes it much easier to count the number of atoms. However, there are a few assumptions we have to make before we publish the calculator.

First, we must assume that all atoms are inside stars, even though they are not. Unfortunately, we have a less accurate idea of ​​how many planets, moons, and space rocks there are in the observable universe than stars, which means it’s hard to add to the equation. But since the vast majority of atoms in the universe are contained within stars, we can get a good approximation of the number of atoms in the universe by just knowing how many atoms there are in stars and ignoring everything else.

Second, we must assume that all atoms in the universe are hydrogen atoms, even though they are not. Hydrogen atoms make up about 90% of the total atoms in the universe, according to Los Alamos National Laboratory, and an even higher percentage of atoms are in stars, which we focus on. As you will soon see, it makes the calculations a lot simpler.

do calculations

Now, it’s finally time to do the math.

To calculate the number of atoms in the observable universe, we need to know its mass, which means we have to know how many stars there are. There are about 10^11 to 10^12 galaxies in the observable universe, and each galaxy contains between 10^11 and 10^12 stars, according to the European Space Agency. This gives us between 10^22 and 10^24 stars. For the purposes of this calculation, we can say that there are 10^23 stars in the observable universe. Of course, that’s a best guess; Galaxies can range in size and number of stars, but since we can’t count them individually, we’ll have to do so for now.

On average, a star weighs about 2.2 x 10^32 pounds (10^32 kg), according to Science ABC, which means that the mass of the universe is about 2.2 x 10^55 pounds (10^55 kg). Now that we know the mass, or the amount of a substance, we need to know how many atoms it has. On average, each gram of matter contains about 10^24 protons, according to the Fermilab National Particle Physics Laboratory in Illinois. This means that it is the same number of hydrogen atoms, because each hydrogen atom contains only one proton (which is why we made an earlier assumption about hydrogen atoms).

This gives us 10^82 atoms in the observable universe. To put that into context, that’s 100,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000,000 atoms.

This number is only a rough guess, based on a number of estimates and assumptions. But given our current understanding of the observable universe, it is unlikely that it would be too far from reality.

Originally published on Live Science.

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