Open universe: One that continues to expand. Gravity slows the rate of expansion but is not strong enough to stop it.
Closed universe: One that will eventually collapse back on itself. This would result in a BIG CRUNCH which is the reverse of the Big Bang.
Flat universe: The force of gravity keeps slowing down the expansion but theoretically, it'll take an infinite amount of time for it to come to rest.
E.4.9 Define the term critical density by reference to the flat model of the development of the universe.
Critical density: The theoretical value of density that would create a flat universe. The value of critical density is estimated to be 4.5 x 10^-27kgm^-3 but is not certain.
15) a) Actual density < Critical density: Open universe
Actual density > Critical density: Close universe
Actual density = Critical density: Flat universe
b) i) 
= 3 x ((2.7 x 10^-18)^2)/ (8 xπx 6.67x10^-11)=1.3 x 10^-26
ii) Determining equivalent no. of nucleons per unit volume:
Density = mass / volume
1.3 x 10^-26=
E.4.10 Discuss how the density of the universe determines the development of the universe
As stated above
E.4.10 Discuss how the density of the universe determines the development of the universe
As stated above
Actual density < Critical density: Open universe
Actual density > Critical density: Close universe
Actual density = Critical density: Flat universe
E.4.11 Discuss the problems associated with determining the density of the universe.
- Remember that research in this area is ongoing and complex
- In common with many other aspects of the universe, much about this phenomenon is not well understood.
DARK MATTER: Matter that we cannot observe because it is not radiating sufficiently for us to observe it. Much of the mass of the universe itself is dark matter, in fact, we can only see a maximum of 10% of the matter which exists in the galaxy.
Why is there so much dark matter?
MACHOs: Matter can be found in MACHOs, stands for Massive Astronomical Compact Halo Objects. There is some evidence that lots of ordinary matter does exist in these groupings. These can be thought of as low-mass 'failed' stars or high mass planets. They could even be black holes which means that they would produce little or no light.
WIMPs: There could be new particles we do not know about, WIMPs, stands for Weakly Interacting Massive Particles. Many experimenters around the world are searching for WIMPs.
So why is the density of the universe so difficult to determine?
- We can only see 10% of the universe
- Most of universe is made up of dark matter - too cool to be detected
- Dark matter may be in the form of MACHOs or perhaps yet to be discovered WIMPs
E.4.12 State that current scientific evidence suggests that the universe is open.
In 1997, cosmologists discovered that supernova explosions in distant galaxies showed that instead of the expected deceleration in the rate of the expansion of the universe, it appeared that the expansion was in fact accelerating. So the universe appears to be more open than expected. There must be some other previously unknown force, acting in opposition to gravity, which is pushing the universe apart. THIS NEW PHENOMENON IS CALLED 'DARK ENERGY'.
E.4.13 Discuss an example of the international nature of recent astrophysics research.
Useful web links:
E.4.13 Evaluate arguments related to investing significant sources into researching the nature of the universe.
Summarise the arguments for and against astrophysics research:
FOR:
Understanding the nature of the universe helps shed light on philosophical questions such as Why are we here? and Is there other intelligent life in the universe?Fundamental interesting area of mankind as a wholeHas the potential of improving quality of life for human beings because of subsequent technological developments Our planet may become inhabitable in the futureAGAINST:
Opportunity cost- money could be better spent on providing food, shelter and medical care to the millions of people suffering from hunger, homelessness and disease If money is to be allocated to research, we might be able to get better returns from medical research Better to fund a great deal of small diverse research than putting lots of funding into one field Is the information gained really worth the cost?Sources: Heinemenn HL Physics Book by Chris Hamper and IB Physics Study Guide by Tim Kirk
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