Question

What do we mean when we say that the universe is expanding? How does expansion lead to the idea of the Big Bang?

Short answer

The universe is expanding: galaxies move apart; this supports the Big Bang, where the universe began hot and dense.

Step-by-step solution

Understanding Expansion

When we say that the universe is expanding, it means that galaxies are moving away from each other over time. This is comparable to dots on a balloon's surface moving further apart as the balloon inflates. As space itself stretches, the distances between cosmological points increase.

Observational Evidence

The primary evidence for the expanding universe comes from the observation of redshifts in spectra of distant galaxies. Edwin Hubble discovered that galaxies appear to be moving away from us, with more distant galaxies moving faster, indicated by their redshift.

Concept of the Big Bang

The idea of the Big Bang stems from tracing the expansion of the universe backward in time. If galaxies are moving away now, they must have been closer together in the past. This implies that the universe started from an extremely hot, dense state about 13.8 billion years ago.

Connecting Expansion to Big Bang Theory

Expansion supports the Big Bang theory because it suggests that the universe has been stretching and cooling from an initial singularity. The observable evidence of the cosmic microwave background radiation further substantiates the theory, providing a snapshot of the early universe shortly after the Big Bang.

Key concepts

Big Bang Theory

The Big Bang Theory is the prevailing cosmological model explaining the origin of the universe. According to this theory, the universe began approximately 13.8 billion years ago from a singular, extremely dense and hot point known as a singularity. This initial state then rapidly expanded—a process we refer to as the 'Big Bang.'

During this expansion, all the energy, matter, and space-time itself spread outwards, shaping the cosmos as we know it. It's important to note that the Big Bang was not an explosion in space, but rather an expansion of space itself. One way to visualize this is by imagining the universe as a balloon being inflated; all points on the balloon's surface move away from each other as the balloon expands.

This expansion continues today and is supported by various astronomical observations including the redshift of distant galaxies and the presence of cosmic microwave background radiation.

Cosmic Redshift

Cosmic redshift is a critical observational phenomenon that supports the theory of an expanding universe. When light from distant galaxies reaches us, it is stretched to longer wavelengths due to the expansion of space. This stretching causes the light to shift towards the red end of the spectrum, hence the name "redshift."

Edwin Hubble discovered that the more distant a galaxy is, the faster it appears to be receding from us. This relationship between distance and velocity—known as Hubble's Law—suggests that the universe is indeed expanding. Redshift serves as a fundamental tool for astronomers to measure this expansion and infer distances to far-off galaxies.

It's crucial to understand that redshift is not due to the movement of galaxies through space, but rather the expansion of space itself carrying them further apart. Each galaxy moves away from every other galaxy, offering a uniform observation of redshift in all directions from any vantage point.

Cosmic Microwave Background Radiation

Cosmic Microwave Background Radiation (CMBR) is another critical piece of evidence for the Big Bang Theory. Discovered accidentally in 1965, CMBR is the thermal radiation left over from the time of the early universe, about 380,000 years after the Big Bang when atoms first formed.

This radiation fills the universe almost uniformly and is detected as a faint glow spread across the sky. The temperature variations in the CMBR are very subtle, only a few millionths of a degree, but they offer crucial insights into the conditions of the early universe and the formation of cosmic structures.

• CMBR provides a "snapshot" of the infant universe, helping cosmologists understand its subsequent evolution.
• It confirms the universe's homogeneity on vast scales and supports the inflationary model of the Big Bang.

Through these observations, CMBR supports the notion that the universe has a finite age and has evolved into its current state from a primordial hot and dense phase.