Is redshift moving away?

The short answer is yes, redshift shows that distant galaxies are moving away from us. Redshift is a phenomenon where light from distant galaxies appears more red than expected, indicating that those galaxies are moving away and the universe is expanding. This expansion of the universe was predicted by the Big Bang theory and observations of redshift in the early 20th century provided evidence for this theory. So redshift is direct proof that the universe is expanding and that distant galaxies are rapidly moving away from us.

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What causes redshift?

Redshift is a phenomenon where light from distant objects appears more red than expected. This occurs because the wavelength of light stretches and shifts toward the red end of the spectrum as the light source moves away from the observer.

There are a few key factors that cause redshift:

The expansion of space – As space expands, it stretches the wavelengths of photons traveling through it. This cosmological redshift was predicted by theoreticians and indicates that the universe is expanding.
Relative motion – If a light source moves away from the observer, the light waves are stretched, shifting the light to longer, redder wavelengths. This Doppler redshift is similar to the Doppler effect for sound waves.

Gravitational effects – Strong gravitational fields near massive objects can cause gravitational redshift, where photons lose energy escaping the gravitational field.

For distant galaxies, the primary cause of redshift is the expansion of space. As galaxies move farther apart, the ongoing expansion stretches the wavelengths of the photons emitted from those galaxies, shifting them toward the red end of the spectrum by the time they reach telescopes on Earth. This cosmological redshift provided early evidence for the Big Bang theory and indicates that the universe is still expanding today.

When was redshift discovered?

The phenomenon of redshift was first identified in the 1840s by Austrian physicist Christian Doppler, who proposed the Doppler effect to explain shifts in wavelengths for moving light and sound sources. However, it took several more decades to associate this with measurements from astronomical sources.

Some key events in the discovery and understanding of redshift:

1868 – British astronomer William Huggins used spectroscopy to measure redshifts from galaxies, suggesting they were receding.
1912 – American astronomer Vesto Slipher systematically measured substantial redshifts from spiral nebulae, providing early evidence these were independent star systems later called galaxies.

1917 – Albert Einstein incorporated redshift into his general theory of relativity as a fundamental aspect of an expanding universe.
1929 – Edwin Hubble measured distance to other galaxies and conclusively showed more distant galaxies had greater redshift, indicating the universe is expanding.

So although the basic mechanism of redshift was identified in the mid-1800s, it took over half a century to gather conclusive observational evidence from astronomy and recognize redshift as a signature of an expanding universe. This understanding was culminated in the late 1920s with Hubble’s observations.

How does redshift show the universe is expanding?

Redshift provided early direct evidence that the universe is expanding in the following ways:

More distant galaxies show greater redshift – Hubble observed that the amount of redshift increases with distance, with more distant galaxies shifted to longer, redder wavelengths. This indicates they are moving away faster.
All directions show redshift – Galaxies in all regions of space show redshift, not just on one side of the sky. This indicates a uniform expansion in all directions consistent with a spreading out of space itself.

Matches predicted expansion – The measured expansion rate from redshift matched theoretical predictions for the growth of the universe over time based on general relativity equations.

These key redshift observations matched the predictions from theoretical models of an expanding universe starting from a dense, hot initial state – now called the Big Bang theory. This provided compelling evidence that space is expanding everywhere, stretching light from distant galaxies toward the red, and carrying galaxies along with the expansion.

Modern measurements of even more distant galaxies continue to show this ever-increasing redshift the farther we look, providing confirmation of the ongoing expansion of the universe.

How is redshift measured from galaxies?

Redshift is measured from the light emitted by distant galaxies and other astronomical objects using spectroscopy. Spectroscopy spreads out light into a spectrum of wavelengths for detailed analysis.

There are a few main methods used:

Optical spectroscopy – Galaxies emit light across the optical wavelengths humans can see. By spreading this into a spectrum, researchers can identify patterns of lines corresponding to elements and compounds. The shift of these spectral lines compared to a rest frame indicates redshift.

Radio spectroscopy – Radio telescopes can detect emission from molecules like hydrogen in distant galaxies. The wavelength of spectral lines like the 21-centimeter hydrogen line can be measured to give redshift values.
Photometric redshift – The overall color of galaxies shifts to redder hues with increasing redshift. Photometry across optical and infrared bands can estimate a photometric redshift based on this color change.

Spectroscopic approaches provide the most precise redshift measurements. But photometric redshift estimates are easier for large galaxy surveys where spectroscopy would be too observationally expensive.

Modern projects like the Sloan Digital Sky Survey have measured redshifts for over 3 million galaxies through huge spectroscopic campaigns. This 3D map of the universe provides detailed insights into the expansion history down to faint, extremely redshifted galaxies in the early universe.

What is the current observable limit of redshift?

The most distant observable galaxies measured to date have redshifts around z = 11-12. This corresponds to galaxies that emitted their light when the universe was only a few hundred million years old, in the earliest epochs of galaxy formation after the Big Bang.

Some key records for observed cosmic redshift include:

z = 11.1 – Galaxy GN-z11, the current spectroscopically confirmed record holder at 13.4 billion light years away.
z = 11.9 – Galaxy candidate UDFj-39546284 potentially at 13.6 billion light years, based on photometric redshift estimates.
z = 12.12 – GN-z13 galaxy with an estimated photometric redshift at 13.4 billion light years and 800 million years after the Big Bang.

However, astronomers believe galaxies should exist at even greater distances and redshifts. The James Webb Space Telescope, launched in 2021, is able to observe deeper into the infrared to detect more highly redshifted early galaxies. JWST could potentially identify galaxies at redshifts z = 15 or beyond, corresponding to when the first galaxies formed a few hundred million years after the Big Bang.

But increased redshift also makes distant galaxies incredibly faint and challenging to detect unambiguously. The observable redshift limit will likely only increase modestly in the next few years before hitting a ceiling due to technical constraints.

Does redshift have a maximum possible value?

In theory, there is no hard limit or maximum redshift. If galaxies existed shortly after the Big Bang itself, they would have immense relative velocities and huge redshifts.

However, a few factors make extremely high redshifts impossible to detect:

Age of the universe – Galaxies obviously couldn’t form before the universe existed. The observable universe is estimated at 13.8 billion years old. So the maximum redshift corresponds to the first epoch of galaxy formation, likely within the first few hundred million years after the Big Bang.
Stretched wavelengths – High redshift photons end up with extremely long redshifted wavelengths that become difficult or impossible to detect. Their emission falls out of observable bands.

Faintness – The higher the redshift, the fainter and harder to detect a galaxy is since its light is stretched and diluted. A redshift 20 galaxy would be incredibly challenging to identify.

So while redshift itself has no limit, the practical observable limit will likely top out around z = 15-20. Earlier galaxies at higher redshifts essentially become impossible to unambiguously identify given limitations in technology and their extreme faintness.

What does redshift tell us about the fate of the universe?

The ongoing expansion measured through redshift observations provides insight into the potential long-term fate of the universe.

Redshift indicates that the expansion is accelerating, driven by dark energy making up 68% of the universe’s mass-energy. So the universe’s ultimate fate depends on the nature of dark energy:

Accelerated expansion forever – If dark energy remains constant or strengthens over time, it will drive galaxies apart faster and faster indefinitely. This results in a “Big Rip” scenario where galaxies are ultimately ripped apart.
Eventual slowing expansion – If dark energy weakened over billions of years, the expansion would slow and potentially even reverse resulting in a “Big Crunch” as the universe collapses back on itself.

Steady accelerating expansion – The best current measurements indicate this scenario where dark energy drives steady accelerating expansion over billions of years, and galaxies remain observable but become increasingly redshifted.

Continued precision redshift measurements of very distant supernovae and galaxies will help reveal more about the nature of dark energy and if the expansion rate is truly constant over cosmic timescales. This will allow cosmologists to model the long-term trajectory of the universe.

Conclusion

In summary, redshift provided the first conclusive astronomical evidence that the universe is uniformly expanding in all directions. The steady increase in redshift with distance matched predictions for the growth of space over time. Ongoing redshift research continues to support and refine our understanding of the expansion history of the universe back to the earliest observable epochs. High redshift galaxy observations provide a glimpse at early galaxy formation and conditions in the nascent universe just a few hundred million years after the Big Bang. Redshift remains one of the most important tools in cosmology for probing the origins, evolution, and eventual fate of our vast and accelerating universe.