6.2.3 The Universe

Learning Objectives

Core

1. Know that the Milky Way is one of many billions of galaxies making up the Universe and that the diameter of the Milky Way is approximately 100,000 light-years
2. Describe redshift as an increase in the observed wavelength of electromagnetic radiation emitted from receding stars and galaxies
3. Know that the light emitted from distant galaxies appears redshifted in comparison with light emitted on the Earth
4. Know that redshift in the light from distant galaxies is evidence that the Universe is expanding and supports the Big Bang Theory.

Supplement

5. Know that microwave radiation of a specific frequency is observed at all points in space around us and is known as cosmic microwave background radiation (CMBR)
6. Explain that the CMBR was produced shortly after the Universe was formed and that this radiation has been expanded into the microwave region of the electromagnetic spectrum as the Universe expanded
7. Know that the speed v at which a galaxy is moving away from the Earth can be found from the change in wavelength of the galaxy’s starlight due to redshift
8. Know that the distance of a far galaxy d can be determined using the brightness of a supernova in that galaxy.
9. Define the Hubble constant H0 as the ratio of the speed at which the galaxy is moving away from the Earth to its distance from the Earth; recall and use the equation

10. Know that the current estimate for H0 is 2.2 × 10^–18 per second

11.  Know that the equation

represents an estimate for the age of the Universe and that this is evidence for the idea that  all the matter in the Universe was present at a single point.

Contents to learn

The Universe

• The Universe contains many billions of galaxies.
• The Milky Way is one of the galaxies in the universe.
• Its diameter is approximately 100,000 light years.
• There are three types of galaxies
•                     Spiral
•                    Elliptical
•                    Irregular
• Our Milky Way galaxy is a spiral galaxy

Doppler Effect

• The Doppler effect can be observed in various types of waves, including sound waves, light waves, electromagnetic waves and water waves.
• A stationary object will make sound travel in all directions.

• If the object is moving, then the apparent change in the frequency of waves due to relative motion between the source and observer is called the Doppler effect.
• When the object is moving toward the stationary observer, the waves are compressed, causing the wavelength to decrease and the frequency to increase.

• When the object is moving away from the observer, the waves are stretched, causing the wavelength to increase and the frequency to decrease.

• The Doppler effect is also applicable to light.
• If an object like a star or galaxy is approaching us, it is called a blueshift.
• If an object like a star or galaxy is moving away from us, it is called redshift.

Red Shift

• In redshift, the wavelength of electromagnetic radiation increases when emitted from stars and galaxies moving away from us.
• The greater the distance between the source and the observer, the greater the redshift will be.
• Light emitted from distant galaxies appears redshifted when compared to light emitted from the same source on Earth.
• All galaxies in our universe exhibit redshift.
• All matter in the universe emerged from a single point and has been expanding since then. This is known as the Big Bang theory.
• Redshift is evidence of the Big Bang theory indicating that the universe is expanding away from us.

The Big Bang Theory

The Big Bang theory is the fundamental cosmological model for the origin and evolution of the universe.

• According to this theory, the universe expanded from a single point of high density and temperature approximately 14 billion years ago and has been expanding and cooling ever since.
• All matter, energy, space and time were created in an event known as the Big Bang, and the expansion of the universe continues to this day.

Cosmic Microwave Background Radiation (CMBR)

• The Big Bang produced radiation energy which exists in the universe as Cosmic Microwave Background Radiation (CMBR) of a certain frequency.
• CMBR fills the entire universe with similar intensity and is observed at all points in space around us.
• As the universe expanded over the years, this caused the CMBR to redshift into the microwave region of the electromagnetic spectrum.
• CMBR is represented by the following map

• The cosmic microwave background radiation map shows areas of higher and lower temperatures.
• The blue area represents lower temperature and the orange/red area represents higher temperature.
• Places with higher temperature have a higher concentration of galaxies, Sun and planets while places with cooler temperature have a low density of galaxies, Sun and planets.

Hubble’s Constant

• Hubble’s constant (H_o) is a fundamental parameter in space physics that describes
•                 The rate at which the universe is expanding.
•                 The relationship between the distance to a distant galaxy and its recessional velocity which is directly proportional.
•                  Provides information about the age, size, and future evolution of the universe.
• The Hubble constant is defined as the ratio of the speed at which a galaxy is moving away from Earth to its distance from Earth.

Where,

v = speed at which a galaxy is moving away from Earth and is found from the change in the wavelength of the galaxy’s starlight due to redshift

d = distance of a faraway galaxy and is found using the brightness of a supernova in that galaxy

Calculation of Hubble’s constant

• The graph between recession velocities of galaxies and the distance of galaxies from Earth is shown in the figure
• From the graph
•                       The speed at which galaxies are moving away from Earth is directly proportional to their distance.
•                      More distant galaxies are moving away from Earth at a faster speed while galaxies  closer to Earth are moving                         away at a slower speed.
• The gradient of the graph is the Hubble’s constant and its estimated value is H_o = 2.2 x 10^-18 per second.

Calculating the age of the Universe

Since Hubble’s Law states that

Inverse both sides of the equation

• This time is approximately equal to 14 billion years.
• This is the estimated age of the Universe and it is evidence for the idea that all the matter in the Universe was present at a single point and continuously expanding.