What are redshift and blueshift? How astronomers learn the distance of distant objects

When astronomers fix the deep universe or the spectra of a distant star subtly hiding an exoplanet, they must account for its movement. This relies on two phenomena – redshift and blueshift.

As objects move away from us, their light is shifted to longer wavelengths or to the red end of the spectrum – this is redshift. Blueshift is the opposite, when light is shifted to shorter wavelengths on the blue side of the spectrum when an object is coming towards us. These give vital clues about things like distance – and when you’re looking at a distant galaxy, it lets you know how close you are to looking at the dawn of time. This is important for telescopes like the James Webb Space Telescope, which its stakeholders tasked with learning about the earliest galaxies in the nascent universe.

To learn more about redshift and blueshift, Reverse spoke with Salvatore Vitale, assistant professor of physics at the Massachusetts Institute of Technology. Vitale analyzes gravitational wave data (ripples in spacetime) mapped by the Laser Interferometer Gravitational Wave Observatory (LIGO) following huge events like black hole mergers. LIGO physicists work with astrophysicists to plot the distance to the gravitational waves they measure using redshift and blueshift – drawing on multiple paths to understand the merging of some of the most powerful forces. powerful in the universe.

This NASA visualization shows how something gradually reddens as it moves away from Earth. Nasa

Understanding blueshift and redshift

While redshift and blueshift sound esoteric, Vitale said we experience it in everyday life with the sirens of ambulances and police cars. Sound and light are associated with waves, so the analogy works: “Sound has a different distance to travel when the car is coming towards you, instead of away from you,” says Vitale. Reverse.

As the siren approaches, the sound increases in frequency and as it moves away, the sound decreases in frequency. This is more commonly known as the Doppler effect, which is the apparent difference between the frequency of the waves an observer experiences relative to the source of the waves. The difference in motion between the observer and the source of the waves creates this effect.

When viewing objects in visible light, he added, “The universe has been kind to us.” Atoms always have characteristic frequency rates, which means we can tell (for example) exactly how often hydrogen in stars should be in a lab. So if the hydrogen observed in a star system has a lower frequency, the star system moves away, and vice versa.

Redshift and the expansion of the universe

Usually, the redshift is discussed when talking about the expansion of the universe. An event 13.8 billion years ago, dubbed the Big Bang, caused the rapid inflation and expansion of spacetime. Astronomers are still seeing the echoes of this Big Bang, as objects in the universe are all moving away from each other, experiencing some degree of redshift.

The most distant objects have the highest redshift. We know that the universe is accelerating thanks to the measurement of the redshift of a particular type of star explosion (supernova), called Ias. Astronomers have dubbed these types of supernovae “standard candles” because they have constant brightness. Since we know the inherent brightness of these supernovae, we can then plot their brightness in association with distance.

The surprise came in 1998, when astronomers announced that supernovae were receding much faster than expected. Before that, astronomers assumed that the universe was expanding at a constant rate. This led to the realization that the universe is getting faster as it expands, based on two independent studies of supernovae involving the Hubble Space Telescope and many other observatories to make sure they saw things properly.

Why the universe is accelerating as it goes is up for debate, but the main hypothesis is a theoretical force called “dark energy”. Astronomers have called the energy “dark” because we can’t feel it with our conventional telescopic instruments that look at waveforms of light. But we can measure the effect of energy, because we can see that the expansion of the universe is accelerating.

Although we cannot determine why the acceleration occurs, the discovery of this phenomenon gave the discovery teams a Nobel Prize in 2011.

As the James Webb Space Telescope prepares for first light, astronomers are especially excited to use its infrared capabilities to peer into the early universe on high redshift objects, those that will come soon from time after a period called the epoch of reionization – when the first galaxies gave the universe a transparent glow.

HD1 is a candidate galaxy that may be the most distant galaxy we have ever seen – a relic of the early universe. Harikane et al.

LIGO and redshift

LIGO is a planet-sized array of telescopes that maps huge events in spacetime and also relies on redshift measurements. In October 2017, astronomers made the first confirmed detection of gravitational waves. They saw the effect of two neutron stars colliding – or the dense remnants of city-sized stars left behind after supernova explosions.

While the gravitational wave aspect of the initial discovery receives the most attention, what’s also important to astronomers is that they mapped this event into visible light waves, Vitale says. For the same event, he said, “you can use the light from the source to get the redshift, and you can use the gravitational waves from the source to get the distance.” For this event and other gravitational waves, in other words, we have to use the redshift to understand the distance.

How do we measure this distance? It goes back to Albert Einstein’s equations of 1916, when his theory of general relativity explained how massive objects like these neutron star collisions could warp the fabric of spacetime. Supercomputers in the 1990s finally allowed researchers to precisely map these distortions. Astronomers were able to model the field equations predicted by Einstein.

Today, it is common for gravitational wave scientists to model different types of mergers (such as between neutron stars or between black holes) at different distances. Then, when these events are observed in the real world, they have “waveforms” or patterns of wavelengths that they can use to match what they see with the predicted distance. This is what happened successfully in 2017.

The distance and redshift thus allow astronomers to measure the expansion of the universe, which is the underlying reason why this gravitational wave event was so significant that the teams were awarded a Nobel Prize for their work. .

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