Big bang happened about 13.7 billion years ago in our universe.
<h3>
Describe the beginning of the universe according to the big bang theory?</h3>
According to the big bang theory, about 13.7 billion years ago, an explosive expansion began, expanding our universe outwards faster than the speed of light.
<h3>Describe the future of the universe according to the flat model?</h3>
According to the flat model, the universe is infinite and will continue to expand forever because the universe is expanding.
<h3>What is cosmic background radiation? </h3>
Cosmic background radiation is a weak radio-frequency radiation that is traveling through outer space in every direction. It is the residual radiation of the big bang, when the universe was very hot.
<h3>How do observations of the cosmic background radiation provide evidence to support the big bang theory? </h3>
The Big Bang theory predicts that the early universe was a very hot place and that as it expands, the gas within it cools. Thus the universe has all over radiation which is called the “cosmic microwave background".
Learn more about big bang here: brainly.com/question/10865002
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Hot air rises = it's less dense than cold air = falls
Answer:
Therefore,
The frequency heard by the engineer on train 1
Explanation:
Given:
Two trains on separate tracks move toward each other
For Train 1 Velocity of the observer,
For Train 2 Velocity of the Source,
Frequency of Source,
To Find:
Frequency of Observer,
(frequency heard by the engineer on train 1)
Solution:
Here we can use the Doppler effect equation to calculate both the velocity of the source 
and observer 
, the original frequency of the sound waves 
and the observed frequency of the sound waves 
,
The Equation is
Where,
v = velocity of sound in air = 343 m/s
Substituting the values we get
Therefore,
The frequency heard by the engineer on train 1
Answer: Explanation:
Plants and animals each produce the gases that the other needs to live. Plants need carbon dioxide—people and other animals exhale carbon dioxide as a waste product. People and other animals need oxygen—plants produce oxygen during an important process called photosynthesis, which turns the sun's energy into nutrients.
