M23 Problems with the Big Bang Model

Big Bang Model

Is a thoroughly tested model in science with its essentials largely confident

• However, there are a few problems that require solutions

Big Bang Model Problems

1. The Flatness Problem

2. The Horizon Problem

3. The Hubble Tension

4. Why did the Big Bang occur

The Flatness Problem

Our universe is geometrically flat. But why? There are an infinite number of possible ways for the universe to be closed or open but total density is just right to make the universe flat

• Ω = 1

Fine-Tuning Problem

Encompasses the flatness problem

• A broader concept that asks why do the fundamental conditions/constants of the universe have such precise values?

Observable Universe

A spherical region centered on an observer

• Radius is the max distance light could have traveled since the Big Bang

Age and Radius of the Universe

13.8 billion years is the age of the universe. But due to spacetime expansion, the radius is actually 46.5 billion ly

The Horizon Problem

Although opposite sides of our sky have had time to send light to us since the Big Bang, they have not had the chance to share info about their physical conditions

• No Causal Contact

Causal Contact

Two regions can influence each other because a signal (like light) has had time to travel between them

Cosmic Microwave Background

In the CMB, causal contact is observed

• Almost the same temperature/wavelength in every direction

Inflation

A modification to the Big Bang theory that solves both the Flatness and Horizon problems

Inflation Mechanism

Posits that in the first few moments of the universe’s existence, the universe expanded at a vastly greater rate than it currently does

Inflation and Flatness Problem

Any universe experiencing inflation would appear flat, similar to how a small region of Earth’s surface appears flat

• A period of exponential expansion would make any initial curvature virtually impossible to measure in the present day

  • On very small scales, any curved surface looks flat

Inflation and Horizon Problem

Prior to inflation, opposite sides of our sky were in contact with one another

• Inflation dragged them apart at speeds much higher than the speed of light

  • Makes it appear that they were never in contact, and thus unable to communicate

Inflation and Horizon Problem and Relativity

Spacetime moving faster than the speed of light is not a violation of relativity

• It only says that objects moving through spacetime can’t move at or above the speed of light

• Spacetime can move at any speed it wants

Universe Possible Curvature

It’s possible the universe could have a curvature, but our observable universe is only a tiny part of a much bigger whole

Inflation Evidence

Proving inflation happened is very difficult but is still an active area of research

Hubble’s Constant

(H₀) Measures the rate of the universe’s expansion

• There are multiple ways to measure H₀ (just as there are many ways to measure distance in the universe)

The Hubble Tension

The different ways to measure the Hubble constant gave basically the same answer, however, as these methods got more precise, they stopped agreeing within their uncertainties

• Also known as the “Crisis in Cosmology”

Scientific Response to Discrepancy

When predictions disagree with observations our options are:

1. Our model of reality is wrong

2. Our observations of reality are wrong

Hubble Tension Discrepancy Response

After about a decade of work, it has only strengthened the disagreements in the data

Hubble Tension Hypotheses

A clue is that each method measuring H₀ probes different eras in the history of the universe

1. The expansion rate may have changed in an unexpected way

2. Dark energy might be behaving differently

Dark Energy Recent Research

In March 2026 two research collaborations announced results implying dark energy varies in time rather than being a cosmological constant

• Dark energy would be a factor that evolves for reasons not known yet

Future Dark Energy Research

The Euclid Space Telescope and the Nancy Grace Roman telescopes will give us a better idea of how dark energy behaves

• Will help resolve whether dark energy has actually evolved over time, and hopefully resolve the Hubble tension

Why did the Big Bang happen?

Remember that the Big Band Model (ΛCDM, Lambda Cold Dark Matter) explains that approximately 13.8 billion years ago, the universe originated from a hot, dense state and has been expanding since

• The model does not explain what happened prior to this state or why this state came into existence

Big Bang Hypotheses

1. Cyclic Universe

2. Eternal Inflation

3. Black Hole Spawning

Cyclic Universe

Suggests a universe that undergoes an infinite series of Big Bangs and Big Crunches

• The end of one universe marks the beginning of another

Eternal Inflation

Proposes that within a larger “universe",” pockets undergo inflationary periods, effectively spawning new universes, which can spawn more.

• This is unlike our standard model which assumes the entire universe underwent a Big Bang simultaneously (inflation happened everywhere, and just once)

Black Hole Spawning

Suggests black holes could spawn new universes

Current State of Knowledge

The origin of the Big Bang is currently unknown

• This fuels scientific curiosity and wonder