What is the evidence that the universe is expanding, and can you explain the various lines of reasoning used to arrive at this conclusion? Specifically, I’m interested in understanding how observations of redshifted light from distant galaxies play a role, what other independent methods support this finding (e.g., observations of the cosmic microwave background or the abundance of light elements), and how potential alternative explanations for these observations have been ruled out? Furthermore, how do scientists address uncertainties and potential sources of error in these measurements and interpretations?

Answer

The primary evidence for the expanding universe comes from several independent observations:

1. Redshift of Distant Galaxies:

• Hubble’s Law: In the 1920s, Edwin Hubble, along with others, observed that the spectral lines of light from distant galaxies were shifted towards the red end of the spectrum (redshifted). This redshift is interpreted as a Doppler effect, indicating that the galaxies are moving away from us.
• Velocity-Distance Relationship: Hubble discovered a direct proportionality between a galaxy’s distance and its redshift (and therefore its recessional velocity). The further away a galaxy is, the faster it is receding. This relationship is known as Hubble’s Law. Mathematically, it’s expressed as v = H₀d, where v is the recessional velocity, d is the distance, and H₀ is the Hubble constant.
• Cosmological Redshift vs. Doppler Redshift: It is important to note that cosmological redshift, although similar to the Doppler effect, is conceptually different. Cosmological redshift is due to the expansion of space itself, which stretches the wavelength of the light as it travels through the expanding universe. It is not simply the result of galaxies moving through static space.

2. Cosmic Microwave Background (CMB):

• Afterglow of the Big Bang: The CMB is the afterglow radiation from the Big Bang, the event that marked the beginning of the universe. It’s a nearly uniform, faint microwave radiation that permeates the entire universe.
• Expansion and Cooling: As the universe expands, the CMB radiation has been stretched and cooled. Its current temperature of about 2.7 Kelvin (degrees above absolute zero) is consistent with the predictions of the Big Bang model, given the amount of expansion the universe has undergone since the CMB was emitted.
• Anisotropies and Structure Formation: Tiny temperature fluctuations (anisotropies) in the CMB provide crucial information about the early universe. These fluctuations represent the seeds of structure formation, the regions of slightly higher density that eventually collapsed under gravity to form galaxies and clusters of galaxies. The observed patterns of these anisotropies match predictions based on the expanding universe model and our understanding of gravity and particle physics.

3. Abundance of Light Elements:

• Big Bang Nucleosynthesis (BBN): The Big Bang model predicts the relative abundance of the lightest elements (hydrogen, helium, lithium, and their isotopes) that were created in the first few minutes after the Big Bang.
• Observed Abundances: The observed abundances of these elements in the universe, particularly in primordial gas clouds that haven’t been significantly altered by stellar processes, are in remarkable agreement with the predictions of BBN, given the expansion rate of the universe inferred from other observations. If the universe had expanded at a different rate, the nuclear reactions that produced these elements would have resulted in different relative abundances.

4. Evolution of Galaxies and Active Galactic Nuclei (AGN):

• Looking Back in Time: Because light takes time to travel across the vast distances of the universe, when we observe distant objects, we are seeing them as they were in the past.
• Density and Properties: Observations show that galaxies and AGN were more numerous and more closely packed together in the early universe than they are today. This is consistent with the idea that the universe was denser in the past and has expanded over time, spreading these objects further apart.
• AGN Evolution: AGN, which are powered by supermassive black holes at the centers of galaxies, were more common and more luminous in the past. This suggests that the conditions necessary for fueling these black holes were more prevalent in the early universe, likely due to the denser environment and more frequent galaxy interactions. Their declining abundance over time fits with the expansion of the universe.

5. Supernovae as Standard Candles:

• Type Ia Supernovae: Type Ia supernovae are a specific type of exploding star that have a consistent peak luminosity, making them useful as "standard candles" for measuring distances across the universe.
• Accelerating Expansion: Observations of distant Type Ia supernovae have revealed that they are fainter than expected based on their redshift, indicating that they are farther away than they should be if the universe were expanding at a constant rate. This led to the discovery of the accelerating expansion of the universe, driven by dark energy.
• Distance Measurement: By comparing the observed brightness of these supernovae to their intrinsic luminosity, astronomers can determine their distances, and combining this information with their redshift, they can map the expansion history of the universe.

6. Baryon Acoustic Oscillations (BAO):

• Sound Waves in the Early Universe: BAO are fluctuations in the density of baryonic matter (ordinary matter made of protons and neutrons) that arose in the early universe due to sound waves propagating through the plasma before the CMB was released.
• Standard Ruler: These oscillations left a characteristic imprint on the distribution of galaxies, creating a preferred separation distance between galaxies. This characteristic distance acts as a "standard ruler" for measuring distances across the universe.
• Expansion History: By measuring the apparent size of the BAO pattern at different redshifts, astronomers can determine the expansion history of the universe. BAO measurements provide independent confirmation of the accelerating expansion and are consistent with the CMB and supernovae observations.

These diverse and independent lines of evidence all converge on the same conclusion: the universe is expanding and has been expanding for billions of years. The expansion is not just a theoretical construct, but a directly observed phenomenon with far-reaching implications for our understanding of the universe’s past, present, and future.