Cosmology Fundamentals: The Big Bang, Hubble's Law, and the Universe's Fate

What is Cosmology?

Cosmology Defined

Cosmology is the study of the origin, present structure, evolution, and density of the universe.

The Cosmological Principle

The cosmological principle is the basic assumption that physical laws are true everywhere in the universe, not just on Earth.

The Expanding Universe

Galactic Recession

We observe galaxy clusters receding from us, which implies that the universe is expanding. Galaxies farthest from us are moving away the fastest; the farther the distance, the faster the speed.

Hubble's Law

Hubble's Law states that a galaxy’s velocity of recession (v) is directly proportional to its distance from us (d). The formula is v = Hd.

The Hubble Constant

The Hubble constant (H) is a crucial value that gives the rate at which the galaxies are receding, or the rate at which the universe is expanding.

The Big Bang Theory

The Big Bang theory states that approximately 13.7 billion years ago, our universe expanded rapidly from an infinitely hot, dense state and has been evolving ever since. The Big Bang was the beginning of time and space as we know them.

Key Evidence for the Big Bang

• The expansion of the universe, observed through galaxy redshift.
• The existence of Cosmic Microwave Background (CMB) radiation.
• The abundance of primordial elements like hydrogen, helium, and lithium.

Models for the Universe's Fate

• Open Universe Model: The universe will continue to expand indefinitely.
• Flat Universe Model: The universe will continue expanding, but at a slower rate as time increases.
• Closed Universe Model: The universe will eventually stop expanding and collapse back on itself.

Rejected Cosmological Theories

The Steady State Theory

The Steady State Theory was rejected because it contradicts key observations, particularly the Nobel Prize-winning discovery of cosmic background radiation. The theory cannot explain the existence of this radiation, leading to its invalidation.

Cosmic Microwave Background Insights

WMAP Discoveries

The Wilkinson Microwave Anisotropy Probe (WMAP) measured slight temperature variations in the average sky temperature of 2.7K. These are evidence of tiny fluctuations in the nearly uniform density of the early universe. These “ripples” of wispy matter provided the gravitational pull that could have formed the galaxies, clusters of galaxies, and the great voids in space we see today.

Measuring the Universe

Determining the Universe's Age

There are two primary ways to measure the age of the universe:

1. Measure the rate of expansion and extrapolate back to the Big Bang. This is known as Hubble Time (1/H).
2. Date the oldest stars. For example, the Hubble Space Telescope has observed white dwarf stars that are 12-13 billion years old.

Current Universe Estimates

• Age: Approximately 14 billion years old.
• Size (Hubble Radius): Approximately 14 billion light-years (c/H).
• Shape: Flat, according to WMAP data.