Q: Does this mean the galaxies are moving faster than the speed of light? ![]() Light from the earliest galaxies may have traveled 13 billion years to reach us, but those galaxies could be around 45 billion light-years distant by now. That in turn means there’s more space to undergo expansion, so the objects appear to be moving away from us much faster. That’s because the farther away an object is, the more space there is between us and the object. But for galaxies extremely far away, the distance is significant. For nearby galaxies, the expansion doesn’t make much of a difference. If space is expanding, are those galaxies even farther away now? Q: So space is expanding and the light from the earliest galaxies has traveled over 13 billion years to reach us. Imagine this happening in three dimensions instead of on a flat surface, and you can understand why it looks like other galaxies are rushing away. If you cover the balloon with dots, and then inflate it, no matter which dot you pick to represent your position, all the other dots will appear to be moving away from it as the balloon expands. The easiest way to visualize this is to imagine a balloon. Does this mean we’re in the center of the universe? Q: Everywhere we look with the Frontier Fields project, galaxies appear to be moving away from us. Projects like Frontier Fields will help us rule out those models that don’t fit the incoming data. The variations in those models include things like the shape of the universe, the rate at which it’s expanding, the amount of normal matter it contains, etc.Īstronomy is about figuring out how the universe works and narrowing down all those models to the best one, and we still have a long way to go. Astronomers describe galaxies in terms of their redshift because unlike distance, it’s a clear and definite value that’s relatively easy to measure without many errors.Īstronomers have different models of how the universe works, and they can plug the redshift into those models to get the distance to a galaxy – but the distance will differ depending on which model of the universe they use. But distance is notoriously difficult to measure in astronomy.Ĭosmological redshift is a direct measurement of the expansion of space. Light-years are a measurement of distance defined by the time it takes light to travel in a year. Q: Why do we hear the Frontier Fields galaxies described in terms of redshift and light-years? Which is right? Cosmological redshift refers to that change and the measure of that change. The farthest galaxies in the universe would have originally emitted visible and ultraviolet light, but since that light has been stretched as it travels, those galaxies appear to us instead in the form of infrared light. Since red light has a longer wavelength than blue light, the light is said to be “redshifted.” Credit: NASA This stretching shifts light into longer wavelengths. That means that the light wave gets stretched as it travels, like a spring being pulled into a different shape. ![]() When we’re discussing the Frontier Fields project, we’re talking about something more precisely called “cosmological redshift.” The space light is traveling through is expanding. ![]() Q: What does it mean when you talk about a galaxy’s redshift? So we see the farthest galaxies as they appeared in the early universe, because the light that left them way back then is finally reaching us just now. For galaxies, we’re talking millions to billions of light years. The light we see from that star in today’s sky is also four years old. We say that star is four light-years away. Light from the next nearest star, however, needs four years to reach us across space. So Neptune is four light-hours away, and the Sun is 500 light-seconds away. We refer to these distances by the time it takes light to cross them. Light from Neptune needs about four hours to cross the solar system. Light from the Sun needs about 500 seconds, or about eight minutes, to reach us from 93,200 miles (150 million km) away. The greater the distances, the greater the time difference.
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