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Chapter 39: Problem 13

How can Hubble's law hold without the universe having a center?

Short Answer

Expert verified
Hubble's law holds without the universe having a center because it suggests the universe is homogeneous and isotropic - looks the same in all directions from any location. The law means that all galaxies are moving away from each other as space itself expands, not because they are moving away from a specific central point. This gives the appearance that each galaxy is at the 'center' of the expansion, because it sees all other galaxies moving away from it.

Step by step solution

01

Understand the Expansion of the Universe

The universe is not expanding out from a center into space; rather, space itself is expanding. Hence, every point in the universe appears to be at the center. To better understand this, imagine a balloon with several dots on it. As the balloon (representing the universe) inflates, all dots (representing the galaxies) are moving away from each other, with no single dot being the central point from which the expansion happens.
02

Visualising Hubble's Law

The Hubble's Law states that the recession velocity \( v \) with which a galaxy moves away from us is directly proportional to its distance \( d \) from us, represented as \( v = H \cdot d \) where \( H \) is the Hubble constant. This is like the spots on an inflating balloon. The farther away the spots are from each other, the faster they move apart as the balloon inflates.
03

Understanding lack of a Universal Center

The fact that each galaxy sees all others receding from it, does not mean there is a central point from which everything is receding. In fact, it suggests just the opposite: there is no central point in the universe. The universe is homogeneous and isotropic (it looks the same from every location and in every direction). This property is a cosmological principle, and the expansive movement abides by it.

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Most popular questions from this chapter

The \(\eta^{0}\) particle is a neutral nonstrange meson that can decay to a positive pion, a negative pion, and a neutral pion. Write the reaction for this decay, and verify that it conserves charge, baryon number, and strangeness.

Express the Hubble constant in SI units.

What forces would be unified by GUTs?

Why are we unlikely to observe an isolated quark?

Pions are the lightest mesons, with mass some 270 times that of the electron. Charged pions decay typically into a muon and a neutrino or antineutrino. This makes pion beams useful for producing beams of neutrinos, which physicists use to study those elusive particles. In a medical application during the late 20 th century, accelerator centers installed "biomedical beam lines" to test pions for cancer therapy. In these experiments, pions attached themselves to atomic nuclei within cancer cells. The nuclei would literally explode, delivering a "pion star" of cancer-killing nuclear debris. Unfortunately, results were not as encouraging as hoped, and enthusiasm for this technique has waned. The negative pion usually decays into a negative muon and one other particle. The other particle could be a. a proton. b. an antineutrino. c. a neutrino. d. an up quark.
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