Cosmic Expansion and Wavelength Effects
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Questions and Answers

What happens to the wavelength of light from distant galaxies as the universe expands?

  • It decreases.
  • It varies randomly.
  • It increases proportionally. (correct)
  • It remains constant.

    • What does the ratio $\frac{l_0}{l}$ represent in the context of cosmological redshift?

  • The distance of a galaxy from Earth.
  • The age of the universe.
  • The scale factor of the universe at different times. (correct)
  • The speed of recession of a galaxy.

    • How much larger is the current scale factor $R_0$ compared to the scale factor 750 million years ago?

  • 5.3% larger. (correct)
  • 15.7% larger.
  • 1% larger.
  • 10% larger.

    • What is the cosmological redshift?

    <p>The increase in wavelength of light due to the expansion of the universe. (B)</p> Signup and view all the answers

    What does a measured wavelength ratio of approximately 8.6 imply about the volume of space?

    <p>The volume of space is about 640 times larger than it was. (D)</p> Signup and view all the answers

    What determines the actual value of the recession speed ($v$) of a galaxy?

    <p>The density of the universe, the value of $H_0$, and the expansion history. (D)</p> Signup and view all the answers

    Why can the recession speed of some galaxies exceed the speed of light?

    <p>Due to the expansion of space itself, not relative motion. (D)</p> Signup and view all the answers

    What is meant by saying, 'the universe isn’t expanding into emptiness'?

    <p>There is no surrounding space for the universe to expand into. (D)</p> Signup and view all the answers

    What does the expansion of the universe signify about its characteristics?

    <p>The scale factor of the universe is increasing. (D)</p> Signup and view all the answers

    What is dark energy theorized to act like in the context of the universe's expansion?

    <p>An antigravity force that causes universal repulsion. (A)</p> Signup and view all the answers

    What proportion of the universe's average energy density is attributed to dark energy?

    <p>68.5% (B)</p> Signup and view all the answers

    How does the average density of matter in the universe compare to the density of dark energy?

    <p>It is less than dark energy. (A)</p> Signup and view all the answers

    What conclusion can be drawn if the average energy density of the universe is equal to the critical density?

    <p>The universe is infinite and expanding forever. (D)</p> Signup and view all the answers

    What is the current estimated age of the universe?

    <p>13.8 billion years (C)</p> Signup and view all the answers

    What is the significance of the average energy density being precisely equal to critical density?

    <p>It shows the universe is infinite and barely unbounded. (D)</p> Signup and view all the answers

    Which percentage best represents the average energy density of conventional matter in the universe?

    <p>4.9% (D)</p> Signup and view all the answers

    What could happen if the average energy density becomes slightly larger than the critical density?

    <p>The universe would become finite and contract. (B)</p> Signup and view all the answers

    What role does gravitational attraction play in the universe's expansion?

    <p>It can slow down the expansion depending on its strength. (A)</p> Signup and view all the answers

    Flashcards

    Cosmological redshift

    The increase in wavelength of light from distant galaxies as the universe expands.

    Scale factor (R)

    A dimensionless quantity representing the ratio of the current size of the universe to its size at a specific time in the past.

    Infinite universe

    The universe has no boundaries; there's nothing outside of it.

    Wavelength (l)

    The distance between two corresponding points of a wave.

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    Expanding universe

    The universe's scale grows larger over time.

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    Hubble constant (H0)

    The rate of expansion of the universe at the present time.

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    Observable universe

    The portion of the universe that we can observe due to the limitations of time and distance.

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    Critical Density

    The exact density needed for the universe's expansion to neither stop nor accelerate.

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    Dark Energy

    A mysterious force that speeds up the expansion of the universe.

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    Recession velocity

    The speed at which distant galaxies appear to be moving away from us due to the expansion of the universe.

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    Wavelength Ratio

    The measurement of the increase/decrease of wavelengths with respect to the expansion of the universe.

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    Dark Matter

    Mysterious matter that accounts for a significant portion of the universe's mass.

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    Recession speed greater than light speed

    Objects can appear to recede faster than light in the expanding universe, because it's not an object moving. Space itself is expanding.

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    Conventional Matter

    The ordinary matter that makes up stars, planets, and us.

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    Big Crunch

    A hypothetical scenario where the universe stops expanding and starts contracting.

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    Universe's Energy Density

    Average amount of energy per volume of space in the universe.

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    Age of the universe

    The calculated age of the universe, approximately 13.8 billion years.

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    Unbounded universe

    The universe has no edges or limits.

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    Study Notes

    Wavelength and Scale Factor in an Expanding Universe

    • Wavelength of light from distant galaxies increases as the universe expands
    • λ₀ / λ = R₀ / R (λ₀ = current wavelength, λ = wavelength at any time, R₀ = current scale factor, R = scale factor at any time)
    • Cosmological redshift: increase in wavelength with time and increasing scale factor
    • Farther objects have greater redshift due to longer travel time and more significant change in R
    • Current largest measured wavelength ratio for galaxies ~ 8.6, implying a significant increase in space volume

    Recession Speed and Relativity

    • Recession speed can exceed the speed of light for very distant objects
    • Does not violate special relativity because recession speed is not from object's motion relative to space coordinates
    • Expansion of space itself causes objects coordinates to move apart rapidly
    • Observable universe is limited by the light travel time

    Infinite and Unbounded Universe

    • Universe is infinite, without edges
    • Expansion is the increase of the universe's scale factor
    • Not expanding into an empty space

    Critical Density and Dark Energy

    • Gravitational attraction slows expansion
    • Dark energy acts as an antigravity, causing universal repulsion
    • Dark energy density is ~ 68.5% of critical density (ρcc2)
    • Matter density is ~ 31.5% of critical density
    • Dark energy contributes substantially to the universe's expansion acceleration, resulting in continuous expansion.

    Universe's Average Energy Density

    • Average energy density of the universe equals ρcc2 (within observational error)
    • Implies an infinite and unbounded universe (nearly)
    • If energy density were slightly larger, universe would be finite
    • Observational error in average energy density is currently <1%, needing improvement to confirm unboundedness.

    Matter Composition of the Universe

    • Ordinary matter (4.9%)
    • Dark matter (26.8%)
    • Dark energy (68.3%)

    Significance of ρcc2

    • Universe is infinite and unbounded (to within observational error)

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    Description

    Explore the concepts of wavelength changes and scale factors in our expanding universe. This quiz covers the relationship between cosmological redshift, recession speed, and the implications of an infinite universe. Test your understanding of these fundamental aspects of astrophysics.

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