Both effects could contribute to redshift, but only one aligns with our Universe's reality.

In the realm of physics, as in life, various solutions may lead to the same outcome. However, in the actual Universe we inhabit, only one scenario accurately depicts how reality unfolds. Scientists face the daunting task of determining which of these possibilities accurately reflects the nature of our cosmos. Vijay Kumar poses an intriguing question regarding the expanding Universe:

> When we observe a distant galaxy, the light we receive is redshifted either due to the expansion of space or because the galaxy is receding from us. How can we differentiate between cosmological redshift and Doppler redshift? My online searches have yielded no satisfactory answers.

The implications of this inquiry are profound, as understanding the underlying nature of the Universe is at stake. Yet, we must ensure we are not misled by our interpretations.

Observing distant objects in the cosmos provides a wealth of information through their emitted light. Stars radiate light based on their temperature and the nuclear fusion processes occurring within them, producing light reflective of their physical characteristics. The light from countless stars amalgamates to create the visual we perceive from a distant galaxy, reaching us all at once.

This incoming light is laden with information, and astronomers have devised methods to decode it. By utilizing spectroscopy to break down the light into its constituent wavelengths, we can identify specific emission and absorption features against the continuous background. Atoms and molecules absorb or emit light at distinct frequencies, contingent upon their energy levels.

The specific wavelengths of light emitted or absorbed depend on whether an atom is neutral or ionized. Identifying multiple lines from the same atom or molecule helps confirm its presence in the observed system. Notably, the ratios of these wavelengths remain consistent throughout the Universe.

However, while the fundamental properties of atoms and molecules are universal, our observations are not static. The light from celestial objects can experience systematic shifts, altering overall wavelengths while preserving their ratios.

The pivotal question remains: why does this shift occur? Why does light from distant objects uniformly appear to shift, maintaining the same ratio across all observed lines?

The first explanation involves a familiar phenomenon: the Doppler shift. When a wave-emitting object moves toward an observer, the distance between wave crests decreases, resulting in higher observed frequencies. Conversely, if the object moves away, the crests are spaced further apart, yielding lower observed frequencies. This concept is easily recognized in sound waves from vehicles, but it similarly applies to light.

Alternatively, we have the possibility of a cosmological shift. According to General Relativity, a static Universe filled with matter and radiation is impossible. Instead, such a Universe is compelled to either expand or contract.

In an expanding Universe, light emitted from distant sources stretches as space itself expands, leading to redshift. Conversely, contraction would compress the emitted light, resulting in blueshift.

Observations of galaxies reveal that most are not merely redshifted; their redshift is proportional to their distance from us. The greater the distance, the more pronounced the redshift—a relationship that has been consistently observed.

Proposed in the late 1920s by scientists such as Georges Lemaitre, Howard Robertson, and Edwin Hubble, this correlation was regarded as compelling evidence for an expanding Universe. This conclusion gained traction nearly a century ago, and subsequent data has bolstered this view.

Interestingly, there are four potential explanations for the observed redshift-distance relationship:

1. The light from distant galaxies loses energy as it traverses space.
2. Galaxies evolved from an initial explosion, pushing some farther away.
3. Rapidly moving galaxies exhibit higher redshifts, distancing themselves over time.
4. The fabric of space is expanding.

Fortunately, observational methods exist to differentiate among these scenarios, and the data consistently favors one explanation.

The first test examines the surface brightness of distant galaxies. In a static Universe, distant galaxies would appear fainter due to constant photon density, but light would appear redder at greater distances. The Tolman Surface Brightness test reveals that surface brightness diminishes with redshift, debunking the tired light hypothesis.

The explosion hypothesis proposes that galaxies moving away from us in all directions suggest a singular explosion, akin to shrapnel dispersing. However, observational data indicates that if the Universe were expanding, we would expect to find a greater number of galaxies at larger distances, which has been confirmed by deep galaxy counts.

Lastly, to directly test the redshift-distance relationship, we can measure distances using two common methods:

1. Angular diameter distance, based on an object's known physical size.
2. Luminosity distance, inferred from an object's intrinsic brightness.

Observations confirm that the calculations align with the predictions of an expanding Universe rather than a Doppler effect.

Had we lived in a Universe where galaxies were rapidly moving away, we would not expect to find objects exceeding 13.8 billion light-years, given the Universe's age. Yet, we regularly observe galaxies at distances of 20 to 30 billion light-years, with the most distant light originating from the Cosmic Microwave Background at 46 billion light-years away.

It is vital to consider all potential explanations, ensuring we do not misinterpret the data. Rigorous observational tests have consistently ruled out alternative explanations for the redshift of distant galaxies. The expanding Universe remains the sole interpretation that fits the comprehensive data.

Send your questions for Ask Ethan to startswithabang at gmail dot com!

Starts With A Bang is now featured on Forbes and republished on Medium thanks to our Patreon supporters. Ethan is the author of two books, Beyond The Galaxy, and Treknology: The Science of Star Trek from Tricorders to Warp Drive.