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Chapter 26: Problem 13

What is the cosmological principle? How is it justified?

Short Answer

Expert verified
The Cosmological Principle is a presumption that on large enough scales, the Universe appears the same (homogeneous and isotropic) regardless of the observation point. This principle is justified by observations, that no matter which direction we look, the Universe appears roughly the same, and the cosmic microwave background radiation (CMBR) is largely the same in all directions. Further assertion comes from the Copernican Principle, that humans aren't privileged observers of the Universe.

Step by step solution

01

Definition of the Cosmological Principle

The cosmological principle is a notion or theory in cosmology stating that, on a large enough scale, the Universe is homogeneous and isotropic. This means that the Universe, when viewed on large scales, appears the same, regardless of one's location or direction.
02

Justification of the Cosmological Principle - Homogeneity

The argument of Homogeneity is based on the observation that, no matter in which direction we look, from our perspective, the Universe appears roughly the same. This allows us to assume that the Universe at large scales can be considered homogeneous.
03

Justification of the Cosmological Principle - Isotropy

The justification for isotropy comes from the observation of the cosmic microwave background radiation (CMBR). CMBR is largely the same in all directions to within one part in 100,000. This uniformity supports the hypothesis that the Universe is isotropic on a large scale.
04

Further Justification of the Cosmological Principle

In addition to Homogeneity and Isotropy, another justification comes from the Copernican Principle. This principle suggests that humans are not privileged observers of the Universe, implying that the Universe looks the same regardless of the observation point. This principle holds true to a large extent, thereby further validating the cosmological principle.

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Most popular questions from this chapter

. What does it mean to say that the universe is homogeneous? That it is isotropic?

How can astronomers measure the average mass density of the universe?

Imagine your firm, Creative Cosmologists Coalition, has been hired to create a three-panel, folded brochure describing the principal observations that astronomers use to infer the existence of a Big Bang. Create this brochure on an \(8 \frac{1}{2} \times 11\) piece of paper. Be sure each member of your group supervises the development of a different portion of the brochure and that the small print acknowledges who in your group was primarily responsible for which portion.

What would be the critical density of matter in the universe \(\left(\rho_{c}\right)\) if the value of the Hubble constant were (a) \(50 \mathrm{~km} / \mathrm{s} / \mathrm{Mpc}\) ? (b) \(100 \mathrm{~km} / \mathrm{s} / \mathrm{Mpc}\) ?

Estimate the age of the universe for a Hubble constant of (a) \(50 \mathrm{~km} / \mathrm{s} / \mathrm{Mpc}\), (b) \(75 \mathrm{~km} / \mathrm{s} / \mathrm{Mpc}\), and (c) \(100 \mathrm{~km} / \mathrm{s} / \mathrm{Mpc}\). On the basis of your answers, explain how the ages of globular clusters could be used to place a limit on the maximum value of the Hubble constant.
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