![]() Unfortunately, we have a much less accurate idea of how many planets, moons and space rocks there are in the observable universe compared with stars, which means it is harder to add them into the equation. However, there are a few more assumptions we have to make before we break out the calculator.įirst, we must assume that all atoms are contained within stars, even though they aren't. Knowing the observable universe's size and that matter is equally and finitely distributed across it makes it a lot easier to calculate the number of atoms. 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 within stars. In other words, there are no regions of the universe that have more matter than others. 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 cosmos - a concept known as the cosmological principle. This is important because our picture of the observable universe is not a single snapshot in time.Īccording to our observations of the known universe, the physical laws that govern it are the same everywhere. This means matter is finite, so there are the same number of atoms in the observable universe as there always have been, according to Scientific American. But on the cosmic scale of the universe, we can assume that the amount of matter created and uncreated cancel each other out. Related: What happens in intergalactic space?Īccording to Einstein's famous E=mc^2 equation, energy and mass, or matter, are interchangeable, so it is possible for matter to be created from or transformed into energy. The rest consists of dark energy and dark matter, but because they are not made up of atoms, we don't need to worry about them for this mystery. In fact, it makes up only about 5% of the universe, according to NASA. ![]() ![]() Matter is not the only thing in the universe, however. (Image credit: Shutterstock) Cosmic assumptions Notice how the universe has expanded since the Big Bang happened 13.8 billion years ago. We also need to know how much matter, or stuff, is in it. ![]() Using cosmic microwave background radiation, we can work out how fast the universe is expanding, and because that rate is constant - which is currently scientists' best guess (although some scientists think it may be slowing down) - that means that the observable universe actually stretches 46 billion light-years in all directions, according to Live Science's sister site .īut knowing how big the observable universe is doesn't tell us everything we know about how many atoms are in it. But by the time the light reaches us, the galaxy or star is much farther away than it was when we saw it. When we observe a distant galaxy or star, what we are really seeing is where it was when it first emitted the light. But because the universe is constantly expanding, this isn't the case. As it exploded into existence, from a single point of infinite mass and temperature, the universe began expanding outward and hasn't stopped since.īecause the universe is 13.8 billion years old and the observable universe stretches as far away from us as light can travel in the time since the universe was born, you might assume that the observable universe stretches only 13.8 billion light-years in every direction. The universe was created during the Big Bang 13.8 billion years ago. However, it is possible to work out roughly how many atoms are in the observable universe - the part of the universe that we can see and study - using some cosmological assumptions and a bit of math.
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