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Chapter 17: Problem 4

Why can't our current theories describe the history of the universe during the Planck era?

Short Answer

Expert verified
Current theories fail to describe the Planck Era due to the absence of a theory combining general relativity and quantum mechanics.

Step by step solution

01

Define the Planck Era

The Planck Era refers to the time period immediately after the Big Bang, specifically from zero to approximately \(10^{-43}\) seconds after the event. This era is characterized by conditions of extremely high density and temperature, where the effects of both general relativity and quantum mechanics are significant.
02

Limits of General Relativity

General relativity is a theory of gravity that describes the large-scale structure of the universe. During the Planck Era, the universe was extremely small and dense, conditions under which classical general relativity fails to accurately describe gravitational effects, since spacetime would need to be quantized.
03

Limits of Quantum Mechanics

Quantum mechanics effectively describes the physics of the very small, like atoms and subatomic particles. However, integrating it with the gravitational force on the Planck scale is problematic, as quantum mechanics does not currently account for gravity at such conditions.
04

Lack of a Unified Theory

The primary limitation in describing the Planck Era lies in the absence of a unified theory鈥攐r theory of everything鈥攖hat combines general relativity and quantum mechanics. This theory would need to cohesively describe gravity on quantum scales, which current theories cannot achieve.

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Key Concepts

These are the key concepts you need to understand to accurately answer the question.

Big Bang
The Big Bang is the starting point of our universe's history. It's the moment when space and time as we know it began. Scientists believe the universe was once contained in a hot, dense point called a singularity. About 13.8 billion years ago, this singularity rapidly expanded. This wasn't an explosion in space; rather, space itself was expanding.
The Big Bang theory helps explain the existence of cosmic background radiation and how galaxies are distributed across the universe. However, right after the Big Bang, during what we call the Planck Era, conditions were so extreme that our current understanding hits a roadblock.
general relativity
General relativity, proposed by Albert Einstein, is the best theory we have for describing how gravity works. It tells us how massive objects like stars and planets warp the space around them, pulling in smaller objects.
In general, it works wonderfully for large-scale structures in the universe. However, during the Planck Era, when the universe was at an infinitesimally small scale, general relativity alone isn't enough to describe the happenings. At such small scales, the universe's density and temperature were so high that spacetime itself needed a new approach, possibly a quantum one. Yet, we do not yet have a fully developed theory to tackle this.
quantum mechanics
Quantum mechanics is our most successful scientific theory for the very small. It explains the behavior of particles at atomic and subatomic scales, where energy levels are quantized. Quantum mechanics has given us fundamental insights, like the uncertainty principle and quantum entanglement.
While it's great for tiny particles, it struggles to include gravity, which is the domain of general relativity. Yet, during the Planck Era, understanding how gravity works on quantum scales was crucial. Without combining these two theories, we can't fully describe the universe during this nascent period.
unified theory
The dream of a unified theory, sometimes called the "theory of everything," is to combine general relativity and quantum mechanics into one cohesive framework. Such a theory would explain all forces of nature: gravity, electromagnetism, the strong nuclear force, and the weak nuclear force.
Physicists have been searching for this theory to explain phenomena in the Planck Era. String theory and loop quantum gravity are some attempts at achieving this. Unfortunately, a complete, accepted unified theory hasn't been reached yet. Until that happens, the physics of the Planck Era remain partly mysterious.

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Most popular questions from this chapter

Testing the Big Bang Theory. The Big Bang theory is widely accepted because it has successfully predicted many observed characteristics of our universe and because there are no observations that strongly conflict with the theory. In this exercise, you'll consider five hypothetical observations that are not predicted by the Big Bang theory: a. a star cluster with an age of 15 billion years b. a galaxy with an age of 10 million years c. a galaxy at a distance of 10 billion light-years whose spectrum is blueshifted d. a galaxy containing \(90 \%\) hydrogen and \(10 \%\) helium e. evidence for an increase of the cosmic microwave background temperature with time Before you begin, assign the following roles to the people in your group: Scribe (takes notes on the group's activities), Advocate (argues in favor of the Big Bang theory, Skeptic (points out weaknesses in the Big Bang theory, and Moderator (leads group discussion and makes sure everyone contributes). For each observation, discuss whether it (1) could be explained with the Big Bang theory, (2) could be explained with a revision to the Big Bang theory, or (3) would force us to abandon the Big Bang theory. After listening to the Advocate and Skeptic discuss each discovery, the Scribe and Moderator should choose option \((1),(2),\) or (3) and write down your team's reasoning for each observation.

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer. According to the Big Bang theory, most of the helium in the universe was created by nuclear fusion in the cores of stars.

The Moment of Creation. You've probably noticed that, when discussing the Big Bang theory, we never talk about the very first instant. Even our most speculative theories at present take us back only to within \(10^{-43}\) second of creation. Do you think it will ever be possible for science to consider the moment of creation itself? Will we ever be able to answer questions such as why the Big Bang happened? Defend your opinions.

Decide whether the statement makes sense (or is clearly true) or does not make sense (or is clearly false). Explain clearly; not all of these have definitive answers, so your explanation is more important than your chosen answer. The cosmic microwave background is our main source of information about what the universe was like at an age of about 5 minutes.

What is Olbers\(^{\prime}\) paradox, and how is it resolved by the Big Bang theory?
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