How can the observable universe be over 90 billion light-years across when the universe is only 14 billion years old?
In: Physics
A light year is a measure of distance, the amount of distance it takes one photon of light to travel in one year. Light travels at 3.00×10 to the 8th m per second. 14 billion years old is a measure of time, not distance.
Edit: Misunderstood the question, sorry
Because the universe is accelerating away from the center (site of the big bang). We are hurtling towards the eventual heat death of the universe down the entropy highway.
“light years” is a measure of distance- the distance light has traveled in a year of movement.
Perhaps the rate of expansion of the universe is faster than the speed of light and has outpaced our age.
Edit: I agree, my original comment made little sense.
The universe is expanding at something like 72km per second, per megaparsec (about 3.3 million light years). So, something a megaparsec away from you appears to be moving 72km away every second.
You can kind of think of this like when a balloon is inflated and stretches out. You can make two dots on a deflated balloon and as you blow it up, the dots don’t necessarily move, but the space between them gets bigger. The further apart you make the dots, the bigger the space between them will be when the balloon is fully inflated.
So objects that are huge distances away, like other galaxies that are hundreds of millions or billions of lightyears away, have a huge amount of space expanding in between them, and with enough distance that expansion can appear to be moving them faster than the speed of light, creating a universe that is far bigger than light has had time to travel.
I would recommend reading[this](https://medium.com/starts-with-a-bang/how-is-the-universe-bigger-than-its-age-7a95cd59c605) as it explains it quite well.
TLDR: The universe expanding from everywhere, think of a balloon being inflated with dots drawn over it, from each dot, everything appears to be getting further away from each other.
Now assume a planet is travelling at the speed of light in one direction, and another is travelling at light speed in the opposite direction, the distance between them will be twice the speed of light multiplied by time. This could mean that in 1 year, two planets travelling at light speed could be 2 light years apart. This is similar to what happens with the universe, which is why it isn’t correct to just assume that the size of the universe < it’s age in light years.
Because the universe itself is expanding.
Imagine yourself and a bunch of friends standing together in the middle of a gymnasium. You all run away from each other toward the walls, spreading further apart as you run, but as you’re running the entire room just keeps getting BIGGER. You turn around and look back at your friends and they’re all impossibly far away, as the room has expanded in every direction. They only ran 14 meters in a couple seconds, yet they’re now 45 meters away from the center because the room itself expanded. Your friend on the opposite side from you is now 90 meters away. It sounds crazy, but this is how the math of the universe works out.
Source: https://www.forbes.com/sites/startswithabang/2018/02/23/if-the-universe-is-13-8-billion-years-old-how-can-we-see-46-billion-light-years-away/#662b8ba71303
Edit: as fiiiiine points out, you shouldn’t think of the walls in this example. In fact, imagine that you can’t even see the walls. (It’s a big room, not very well lit.) Really it’s the floor that’s stretching out in every direction. And if one of your friends stayed near the center, they’re still pretty close to center. Meanwhile your furthest friend ended quite far away because of all the floor stretching that happened in between you.
Reading (some) of the responses here, so if i started out at light speed towards a planet thats 10 light years away, itd take me longer to reach it due to the space between source and destination expanding with the universe? Why dont i expand with it as well?
Based on the measured light spectrum of the star, scientists will know the distance from the earth to the star.
However, some aspects need to be clarified:
First: The age of the universe according to the Big Bang theory is 14 billion years, but there are galaxies more than 14 billion years away. Why? Because the Universe is expanding. This was given by Edwin Hubble and has been recognized by the academy. See details: [https://en.wikipedia.org/wiki/Edwin_Hubble](https://en.wikipedia.org/wiki/Edwin_Hubble)
An example: an ant crawls on a rubber sheet at a speed of 10m / h. If the rubber expands at 80m / h, the ant’s position will be at 90m. The same applies to objects that are moving in space, and even light with constant speed applies.
Second, how to know an object in the universe 90 billion light-years from Earth?
Traditional measurement: Use “trigonometric in triangle” geometry to measure distances. Specifically: take the earth and another fixed planet (possibly the sun) that we already know the distance. Then measure the angle and use trigonometry to calculate the distance.
However, this way cannot measure stars at a distance = 90 billion light-years away.
Alternatively: use modern technical means to determine the spectrum of light emitted by the star. The light from each star will have a different spectrum depending on the intensity and distance. Based on color, experts can determine the actual brightness of the object, then compare with the light of the star as a standard to calculate the distance to the earth.
In this way, two factors need to be based:
* Edwin Hubble’s Law: the speed of a galaxy’s distance away is proportional to the distance from it to the Earth.
* [Doppler effect](https://en.wikipedia.org/wiki/Doppler_effect)
The stars are moving but the space in between them is also warping, distorting and expanding. You know in the Doctor Strange movie when he’s running down the hall and Mads Mikkelson’s character does magic to make the hall get longer? It’s like that: the end of the hall got further away even though the mirrors at the end of the hall didn’t physically move from where they were.
Things can only MOVE at the speed of light, but space can EXPAND much faster than that.
Side note: this is why Star Trek’s engines go at “warp speed”: the Enterprise doesn’t move faster than light; it moves at normal speed but the engine creates a “warp bubble” which distorts and warps and shrinks space itself so that the Enterprise has less distance to cover (and after the Enterprise has passed by, space expands again and snaps back to normal, making it look like the Enterprise went a much greater distance than it physically did).
I mention Star Trek [because scientists figure we could actually do that in real life if only we could find the right fuel](https://en.m.wikipedia.org/wiki/Alcubierre_drive).
Piggyback question. What the fuck do scientists see when they look 90 billion lightyears away, considering there wasn’t anything there 90 billion years ago?
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