Cosmology and Astrophysics Quiz

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Questions and Answers

What is most likely the origin of the hot gas detected between galaxies that emits X-rays?

Galactic winds from supernovae

Material from the formation of galaxies that failed to create stars (correct)

Interstellar dust that formed stars

Radiation from nearby active galactic nuclei

How does the emission of the 21-cm line facilitate astronomical observations?

It maps the neutral hydrogen gas in space without obstruction (correct)

It provides information about stellar compositions

It measures the temperature of distant galaxies

It helps detect black holes through X-ray emissions

What do spectral lines indicate when radiation interacts with matter?

The distance to the celestial body

The presence of gravitational waves

The unique energy transitions of elements (correct)

The temperature of the emitting bodies

What does the formula for redshift, $z = \frac{\lambda - \lambda_0}{\lambda_0}$, represent in astrophysics?

The change in frequency of light due to velocity differences Signup and view all the answers

What methodology did Hubble and Humason use to compute the distance to galaxies?

Observing apparent brightness and pulsation periods of Cepheid variables Signup and view all the answers

What does the term cosmology primarily study?

The study of the origin, evolution, and fate of the universe Signup and view all the answers

What is the Standard Model of Modern Cosmology known as?

FLRW Model Signup and view all the answers

In the context of observable universe, what is the typical distance among nearest stars?

Around 1 pc Signup and view all the answers

What is the composition of a typical cluster of galaxies?

About 30 galaxies Signup and view all the answers

How are superclusters of galaxies typically separated?

By huge voids Signup and view all the answers

What is the primary method used to obtain positions of galaxies in the Sloan Digital Sky Survey?

Measuring redshift of spectral lines Signup and view all the answers

At what scale does the universe begin to appear smooth?

At scales of around 100 Mpc or more Signup and view all the answers

Which of the following subjects contributes knowledge to cosmology?

General Relativity Signup and view all the answers

What does Hubble's law indicate about the movement of galaxies?

The expansion of space carries galaxies along with it. Signup and view all the answers

What type of redshift is associated with distant galaxies according to Hubble's law?

Cosmological redshift due to the expansion of space. Signup and view all the answers

How does Hubble's law relate to the age of the universe?

It provides a calculation method based on distance and speed. Signup and view all the answers

In Hubble's law equation $v = H_0 d$, what does $d$ represent?

The distance to the galaxy. Signup and view all the answers

What is a key assumption of the Cosmological Principle as it relates to the universe?

The universe is homogeneous and isotropic on large scales. Signup and view all the answers

Why do distances between galaxies within a cluster remain stable?

Gravitational attraction stabilizes the cluster's size. Signup and view all the answers

What does Hubble's law suggest about the relationship between distance and recessional velocity?

Recessional velocity is directly proportional to distance. Signup and view all the answers

What radius does the cosmic light horizon represent?

The distance time light has travelled in 13.4 billion years Signup and view all the answers

What does Hubble's law suggest about the past state of the universe?

The universe was denser Signup and view all the answers

What is an important fact about the expansion of the universe as it relates to galaxies?

Galaxies do not increase in distance due to cluster stability. Signup and view all the answers

Which of the following does Hubble's law apply to?

Expansion of the universe on large scales Signup and view all the answers

What phenomenon causes it to appear that velocity exceeds the speed of light?

Space expansion Signup and view all the answers

What did Alpher and Hermann propose about the universe following the Big Bang?

It was very hot with thermonuclear reactions occurring Signup and view all the answers

According to Dicke and Peebles, what happens to high-energy photons due to the expansion of the universe?

They lose energy and become long wavelength photons Signup and view all the answers

What did Dicke and his team compute regarding photon intensity?

It peaks in microwave wavelengths Signup and view all the answers

What governs the properties of the photons detected in early cosmological studies?

Planck's blackbody law Signup and view all the answers

Which property of the Cosmic Microwave Background (CMB) was most relevant to the identification of mass concentrations like super-clusters of galaxies?

Non-uniformities in the CMB Signup and view all the answers

What was the crucial observation made by Penzias and Wilson regarding the CMB in their work?

They measured radiation that signatures of the Big Bang. Signup and view all the answers

How can space curvature be measured according to the information provided?

By measuring the angles of a triangle formed by three light sources Signup and view all the answers

What aspect of the CMB did the temperature variations from COBE data signify?

Effects of Earth’s motion through the CMB Signup and view all the answers

What was the state of the early universe around 380,000 years after the Big Bang?

It was a high-density fluid of photons, electrons, and ions. Signup and view all the answers

What phenomenon resulted from particle collisions in the early universe?

Sound waves with compressions and rarefactions Signup and view all the answers

What happens to photons emerging from regions of compression in the early universe?

They have a black body spectrum with higher temperatures. Signup and view all the answers

What conclusion can be drawn if the sum of the angles of the triangle formed by light sources is less than 180º?

Space is hyperbolic. Signup and view all the answers

What is the estimated dark energy density ratio, represented as ΩΛ?

0.76 Signup and view all the answers

Which of the following is NOT a candidate for dark energy?

Quark-gluon plasma Signup and view all the answers

What evidence indicates that the universe is expanding at an accelerated rate?

Observations of distant supernovae Signup and view all the answers

How old is the universe estimated to be?

13400 million years Signup and view all the answers

What is the inferred shape of the universe?

Flat Signup and view all the answers

What is the redshift (z) computed from the observed wavelength of 6970Å and the rest wavelength of 6563Å?

0.062 Signup and view all the answers

What is the calculated velocity of recession from a redshift of 0.062?

18030 km/s Signup and view all the answers

Which of the following is NOT included in the composition of the universe?

Electromagnetic force Signup and view all the answers

Study Notes

Introduction to Astronomy: Evidences in Cosmology

Cosmology is a branch of physics studying the origin, evolution, and fate of the universe.
It integrates knowledge from astrophysics, observational astronomy, general relativity, and particle physics.
The standard cosmological model is the FLRW (Friedmann-Lemaître-Robertson-Walker) model, which describes a Big Bang universe as isotropic and homogeneous.

Introduction
Observational Evidence
- Matter distribution
- Hubble Law
- The Cosmic Microwave Background (CMB)
- Cosmic Curvature
What is the universe made of?
- Mass
- Radiation
- Dark energy
Limits and evolution of density
Summary of the session

Main Questions

How old is the universe?
What is the universe made of?
What is the shape of the universe?
What is the size of the universe?
Where does matter come from?
What triggered the formation of galaxies and how did large structures originate?
How many star populations did/do exist?
What is the fate of the universe?

Observational Evidences: Distribution of Matter

Stars (e.g., the Sun): Mass ~ 10³⁰ kg, distance between nearby stars ~ 1 parsec (pc)
Galaxies (e.g., the Milky Way): ~ 10¹¹ stars, length ~ tens of kiloparsecs (kpc)
Clusters of galaxies (e.g., the Local Group): ~ 30 galaxies, typical distance between galaxies ~ hundreds of kiloparsecs (kpc), volume ~ a few cubic megaparsecs (Mpc)
Superclusters of galaxies (e.g., Coma cluster): ~ 10,000 galaxies, length ~ tens of megaparsecs (Mpc), joined by filaments and walls, separated by voids
Foam-like structure: At scales of hundreds of Mpc or more, the universe appears smooth
Sloan Digital Sky Survey: Obtained galaxy positions by measuring redshifts (spectral lines), radius ~ 600 Mpc

Observational Evidences: Other Wavelengths

Microwave: Cosmic Background (examine subsequent slides)
X-rays: Reveal hot gas (tens of millions of Kelvin) between galaxies, possibly remnants from galaxy formation; this is hot gas that failed to create stars
Radio waves: 21-cm emission line (spin-flip of electron in hydrogen atom) allows mapping neutral hydrogen gas in the distant universe (not blocked by dust)

Observational Evidences: The Hubble Law

Photometry vs. Spectroscopy: Bright or dark lines in continuous spectrum due to emission or absorption of light in narrow frequency ranges; each element has a unique spectral line pattern
Spectral line shift: Measured wavelength changes depending on relative velocity between emitter and receptor; this can be Doppler effect or space itself expanding.
Hubble and Humason: Photographed galaxy spectra and calculated redshifts and velocities using Cepheid variables for distance measurement.

Methodology

Hubble and Humason photographed galaxy spectra and computed redshifts and velocities
Distance to galaxies determined by apparent brightnesses and pulsation periods of Cepheid variables within them
Redshift (z) calculated from the relation z = (\lambda - \lambda_0) / \lambda_0 ,where λ is observation wavelength and λ_0 is rest wavelength.
Velocity (v) calculated from v = c[(z+1)^2-1]/[(z+1)^2+1] where c= speed of light

Comments about the Hubble Law

Galaxies' motion isn't through empty space, but from the expansion of space itself.
Redshift is due to wavelength expansion during photon travel through space (space is expanding)
Distance between cluster galaxies doesn't change because of gravitational attraction.
Hubble's law provides the universe's age (using the relationship T=1/H0), with T ~ 13,400 million years old.
We can only observe objects inside the cosmic light horizon.

Comments about the Hubble Law 3 Important Facts

Galaxies aren't moving away, but space is expanding
Photon wavelengths stretch as they travel through expanding space, causing redshift
Gravitational attraction keeps galaxies in clusters stable.

What is the Universe Made of?: Mass Density

Counting galaxies and other measurements reveal an average luminous matter density of ~4.2 x 10⁻²⁸ kg/m³.
Rotation curves show extended dark matter halos around galaxies.
Dark matter candidates include faint/red low-mass stars, massive neutrinos, Weakly Interacting Massive Particles (WIMPs), and massive compact halo objects (MACHOs), including small black holes and brown dwarfs.
Accounting for visible and dark matter, the present-day average matter density is roughly 2.4 x 10⁻²⁷ kg/m³. This value has an uncertainty of roughly 5%.
Luminous matter comprises only roughly 17% of the average density.

What is the Universe Made of?: Radiation Density

Equivalent radiation density needed for comparison with matter density.
Combining Einstein's E=mc² equation with Stefan-Boltzmann's law gives the energy density of photons.
The current CMB temperature (2.725K) yields a radiation density of ~4.6 x 10⁻³¹ kg/m³.

What is the universe made of?: Dark Energy

Astronomers observe a flat universe (k=0) but only observing matter/energy density to make it hyperbolic (k=-1).
Conclusion: Missing energy density that doesn't emit detectable radiation or effect on gravity, therefore it's called dark energy.
Estimated dark energy density, Ω^ = 0.76.
Dark energy candidates include a constant energy filling space homogeneously (cosmological constant) and scalar fields.

Evidence 2: Accelerated Expansion of the Universe

Observations of distant supernovae indicate an accelerating expansion of the universe.
The acceleration implies a component in the universe with negative pressure, commonly referred to as dark energy.

Limits and Evolution of Density

The universe's density evolution is characterized by a transition from radiation dominance to matter dominance to dark energy dominance.
Density values from observations are consistent with a flat universe (k=0).

Exercise

Calculate the:
- Redshift (z) for a galaxy with an observed H-alpha line wavelength of 6970Å and a rest wavelength of 6563Å
- Recession velocity (v) for this galaxy using z
- Distance (d) to the galaxy using v and H0

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Description

Test your knowledge on various aspects of cosmology and astrophysics, including the origins of cosmic phenomena, the methodologies used for galaxy distance calculations, and the implications of Hubble's Law. This quiz covers fundamental concepts such as the 21-cm line emission and the Standard Model of Modern Cosmology.