What is Blueshift?

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At 2.5 million light-years, the Andromeda Galaxy is the closest spiral galaxy to the Milky Way. Astronomers know it is moving toward the Milky Way because its light is "blueshifted". Adam Evans/Wikimedia Commons.

 Astronomy has a number of terms that sound exotic to the non-astronomer. Two of them are "redshift" and "blueshift", which are used to describe an object's motion toward or away from us in space.

Redshift indicates that an object is moving away from us. "Blueshift" is a term that astronomers use to describe an object that is moving toward another object or toward us. Someone will say, "That galaxy is blueshifted with respect to the Milky Way", for example. It means that the galaxy is moving toward our galaxy. It can also be used to described the speed the galaxy is taking as it gets closer to ours. 

How Do Astronomers Determine Blueshift?

Blueshift is a direct result of a property of an object's motion called the Doppler effect, though there are other phenomena that can also result in light becoming blueshifted. Here's how it works. Let's take that galaxy as an example again. It is emitting radiation in the form of light, x-rays, ultraviolet, infrared, radio, visible light, and so forth. As it approaches an observer in our galaxy, each photon (packet of light)  that it emits appears to be produced closer in time to the previous photon. This is due to the Doppler effect and the galaxy's proper motion (its motion through space). The result is that the photon peaks appear to be closer together than they actually are, making the wavelength of light shorter (higher frequency, and therefore higher energy), as determined by the observer.

Blueshift is not something that can be seen with the eye. It is a property of how light is affected by an object's motion. Astronomers determine blueshift by measuring tiny shifts in the wavelengths of light from the object. They do this with an instrument that splits the light into its component wavelengths. Normally this is done with a "spectrometer" or another instrument called a "spectrograph". The data they gather are graphed into what's called a "spectrum." If the light information tells us that the object is moving toward us, the graph will appear "shifted" toward the blue end of the electromagnetic spectrum. 

Measuring the Blueshifts of Stars

By measuring the spectral shifts of stars in the Milky Way, astronomers can plot not just their movements, but also the movement of the galaxy as a whole. Objects that are moving away from us will appear redshifted, while objects approaching will be blueshifted. The same is true for the example galaxy that's coming toward us.

Is the Universe Blueshifted?

The past, present and future state of the universe is a hot topic in astronomy and in science in general. And one of the ways that we study these states is to observe the motion of the astronomical objects around us.

Originally, the universe was thought to stop at the edge of our galaxy, the Milky Way. But, in the early 1900s, astronomer Edwin Hubble found there were galaxies outside of ours (these had actually been observed previously, but astronomers thought that they were simply a kind of nebula, not entire systems of stars). There are now known to be multiple billions of galaxies across the universe. 

This changed our entire understanding of the universe and, shortly after, paved the way for the development of a new theory of the creation and evolution of the universe: the Big Bang Theory.

Figuring Out the Motion of the Universe

The next step was to determine where we are in the process of universal evolution, and what kind of universe we were living in. The question is really: is the universe expanding? Contracting? Static?

To answer that, the spectral shifts of galaxies near and far were measured. In fact, astronomers continue to do this today. If the light measurements of the galaxies were blueshifted in general, then this would mean that the universe is contracting and that we could be headed for a "big crunch" as everything in the cosmos slams back together. 

However, it turns out the galaxies are, in general, receding from us and appear redshifted. This means that the universe is expanding. Not only that, but we now know that the universal expansion is accelerating ​and that it accelerated at a different rate in the past. That change in acceleration is driven by a mysterious force known generically as dark energy. We have little understanding of the nature of dark energy, only that it seems to be everywhere in the universe.