Download Lecture 21-Hot Big Bang

The Hot Big Bang
The Hot Big Bang
Key Concepts
1) In the Hot Big Bang model, the universe
was initially very hot as well as very dense.
2) In the Hot Big Bang model, hydrogen was
initially ionized and opaque.
3) The Cosmic Microwave Background is
a relic of the hot early universe.
Let’s suppose that the universe was
very hot as well as very dense
when it started expanding.
This hypothesis (hot, dense beginning)
is called the Hot Big Bang model.
If the temperature of the early universe was
T > 3000 Kelvin, then hydrogen was ionized.
So what?
If the universe were very hot and dense
more than 13 billion years ago, what
difference would that make today?
A hot, dense early universe leads to a cosmic
background of light that is visible today.
Ionized gases are opaque because they
contain free electrons that scatter photons
of any energy.
Why does this matter?
Dense ionized gases are
opaque. (You can’t see
through the Sun!)
Photons (squiggles) don’t move freely through
space, because they collide with electrons
(purple dots).
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Why does it matter whether the early
universe was opaque?
Hot, dense, opaque objects emit light!
Today, we call hot, dense,
opaque objects that emit light
“stars”.
The universe is NOT opaque today.
We can see galaxies billions of light-years away.
Soon after the Big Bang, the
entire universe was glowing.
Imagine yourself inside a star, surrounded by a
luminous, opaque “fog”, equally bright in all directions.
The early universe was like that –
sort of monotonous, really…
Why the change from an opaque universe to a
transparent one? Gases cool as they expand.
The universe is NOT uniformly glowing today.
The night sky is dark, with a few glowing stars.
(This accounts for the relative unpopularity of
spray deodorants. Woohoo, that’s cold!)
As the hot, dense, ionized hydrogen
expanded, it cooled.
When its temperature dropped below
3000 Kelvin, protons & electrons combined to
form neutral H atoms.
However, light produced earlier, when the
universe was opaque, didn’t simply disappear.
It radiated freely through the transparent
universe, and should be still be visible today
Is there any evidence of this?
The universe became transparent.
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The universe became transparent at a
temperature T ≈ 3000 Kelvin.
…objects at T ≈ 3000 Kelvin produce visible &
infrared light (think “lightbulb filament” or “cool
star like Betelgeuse”).
What do I mean when I talk about the
“temperature of the Cosmic
Background”?
“The temperature of the Sun’s surface is 5800 Kelvin”:
that’s a statement about the typical speed of atoms
and ions.
The cosmic background changed from visible &
infrared light (λ ≈ 0.001 mm) to microwave light
(λ ≈ 1 mm)
Its temperature dropped from 3000 K
to 3 K.
The universe is expanding.
Distance between galaxies increases.
Wavelength of light (distance between
wave crests) increases.
“The temperature of sunlight is 5800 Kelvin”:
that’s a statement of the typical energy of photons.
The wavelength of cosmic background light
has increased by a factor of 1000.
Flashback:
The wavelength of peak emission for a blackbody is
inversely related to temperature.
peak 
0.001
mm
1 mm
Why? Because the universe has expanded by a factor
of 1000 since the time it became transparent.
2,900,000 nm 2.9 millimeter

T
T
Longer wavelength → cooler temperature.
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The universe is filled with a Cosmic
Microwave Background.
As a result of the expansion of the Universe
the cosmic background radiation changed to microwave
Wavelength and its temperature dropped to about 3K.
Is there any evidence for this
Cosmic Microwave Background?
Microwaves are electromagnetic waves
with wavelengths from about
1 millimeter to about 1 meter.
The Cosmic Microwave Background (CMB) is
discussed briefly in Section 5.3 of the textbook.
The CMB doesn’t reappear until Section 8.4.
Microwaves are used in
wireless communication.
The osuwireless WiFi network is at λ = 6 centimeters.
[802.11a protocol]
Microwave ovens use microwaves
with wavelength λ = 12 centimeters
(5 inches). This wavelength is
strongly absorbed by water.
Cellphones work in narrow wavelength bands in
the range λ = 15 → 70 centimeters.
In the early 1960s, the company AT&T wanted
to improve microwave communication.
They hired two astronomers, Bob Wilson
and Arno Penzias, to improve a
microwave antenna at Bell Labs.
Microwaves are also produced by nature.
Wilson & Penzias were plagued by static.
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Wilson & Penzias did everything they could to
eliminate “noise” in their antenna.
The microwave radiation picked up by
Wilson & Penzias was nearly isotropic.
(That is, it doesn’t come from a
single source, like the Sun.)
…including trapping pigeons that
had left “a white dielectric
material” on the antenna.
Conclusion: “static” or “noise” actually came
from outer space, not from pigeon poop.
Because they come from everywhere, the
microwaves from space are called the
Cosmic Microwave Background.
That’s “Background” in the same sense as
“background” or “ambient” noise.
Discovery
• 1965: Penzias & Wilson (Bell Labs)
The Cosmic Microwave Background isn’t behind the
galaxies we see; it is between them and inside them.
It’s ubiquitous.
Consider the spectrum of the CMB.
Measuring the CMB spectrum is
hard to do from the Earth’s surface.
– Mapping sky at microwave wavelengths.
– Found a faint microwave background noise.
– First thought it was equipment problems
(noisy amplifiers, pigeons in the antenna).
– Finally determined it was cosmic in origin.
• Won the Nobel Prize in 1978 for
discovering the Cosmic Background
Radiation.
Astronomers observe the CMB from above the
Earth’s damp atmosphere with artificial satellites.
COBE
WMAP
The Earth’s atmosphere is opaque to
microwaves with λ < 2 centimeters. (Blame the
water vapor!)
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Intensity
What do these orbiting satellites find?
The Cosmic Microwave Background has a
blackbody spectrum.
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