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SCCC 116
Homework 7 - Due 20 April 2006
Prof. Christina Lacey
R & D: 26-20, 27-6, 27-16, 28-1, 28-12, 28-13

R&D 26-20

Do you think it constitutes good science to explain the universe in terms of dark matter and dark energy, neither of which is known or understood?

Dark matter and dark energy, at this time, are simply names to describe things that appear to exist observationally. We have no physical understanding of dark matter or dark energy except for their effects on normal matter. Actually explanations of both dark matter and dark energy exist, but there remains great debate in the astronomical community as to which explanations are correct, if any. However, astronomers have been forced to postulate dark matter and dark energy due to the observations that have contradicted classical theories. Many new things have been discovered recently. Whether dark matter and dark energy are the correct explanations will be determined through more good science and observations.

R&D 27-6

When the Universe was about 100 seconds old, conditions such as temperature and density were just about right for the fusion of protons and neutrons to form helium. 25% of the matter by mass was converted into helium at that time. Not until stars started nucleosynthesis did hydrogen fuse into helium again. But all matter in the Universe contains at least this 25% of helium; any more than this is due to nucleosynthesis.

R&D 27-16

If inflation is correct than the density of the Universe must be at or very near critical density.

R&D 28-1
Why is life difficult to define?

Not all life shares all the major characteristics: reaction, growth, reproduction, and evolution. Life on earth is very diverse, consider the case of a virus.

R&D 28-12
What is generally meant by the phrase ``life as we know it''? What other forms of life might be possible?

``Life as we know it'' refers to the lifeforms that exist on Earth (organic molecules in a water environment). Life could be based on other molecules other than carbon, such as silicon, or life could be carbon-based but have evolved in a manner that is radically different from what we have currently.

R&D 28-13
How many terms of the Drake equation are known with any degree of certainty? Which factor is least well known?

The Drake equation depends on the following variables and the degree with which it is known:

1) rate of star formation- well known
2) fraction of stars with planets- decently well known
3) number of planets in habitable zone- decently well known
4) fraction of suitable planets on which life evolves- not well known
5) fraction of life-bearing planets where intelligence arises- not well known
6) fraction of planets with technological societies- not well known
7) average lifetime of technological society- widest range of values and the least well known

1, 2 and even 3 can be found from observation and extrapolation. 4, 5, and 6 all are based on earth's life since that is the only example we know. 7 is the least well understood.

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Christina Lacey 2006-05-01