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Is our universe infinite or closed? Because the visible Universe is expanding, the most distant visible things are much further away than its estimated 14-billion year age. In fact, the photons in the cosmic microwave background have traveled a cool 45 billion light years to get here. That makes the visible universe some 90 billion light years across.
Originally posted by harrytuttle
reply to post by XPLodER
Here's what I don't get:
If the photons traveled 45 billion years to get here, then how can the Universe only be 13.7 billion years old?
If the photons traveled 45 billion years to get here, and the universe appears to be expanding even then, then how far away are the sources of those photons right now?
What do we really know about the effects of time and distance on the properties of a light wave?
How can we be so sure that light waves traveling for 10 billion years don't naturally "stretch out" to make them appear as though their sources are traveling away from us (Doppler effect)?
Isn't it true that the only information we have to determine whether our Universe is expanding is billions and billions of years old? If true, then how can we conclude that it's STILL expanding, and not contracting?
In Big Bang cosmology, the observable universe consists of the galaxies and other matter that we can in principle observe from Earth in the present day, because light (or other signals) from those objects has had time to reach us since the beginning of the cosmological expansion. Assuming the universe is isotropic, the distance to the edge of the observable universe is roughly the same in every direction—that is, the observable universe is a spherical volume (a ball) centered on the observer, regardless of the shape of the universe as a whole. The actual shape of the universe may or may not be spherical. However, the portion of it that we (humans, from the perspective of planet Earth) are able to observe is determined by whether or not the light and other signals originating from distant objects has had time to arrive at our point of observation (planet Earth). Therefore, the observable universe appears from our perspective to be spherical. Every location in the universe has its own observable universe which may or may not overlap with the one centered around the Earth.
How can we be so sure that light waves traveling for 10 billion years don't naturally "stretch out" to make them appear as though their sources are traveling away from us (Doppler effect)?
A flat Universe would also be infinite and their calculations are consistent with this too. These show that the Universe is at least 250 times bigger than the Hubble volume.
We show that gravitational lensing by foreground galaxies will lead to a higher number of galaxies to be counted at redshifts z>8-10. This number may be boosted significantly, by as much as an order of magnitude. If there existed only three galaxies above the detection threshold at redshifts z>10 in the Hubble field-of-view without the presence of lensing, the bias from gravitational lensing may make as many as 10-30 of them visible in the Hubble images," explains Windhorst
Something Is Ripping The Universe Apart
Recently the composition of the universe has become even more puzzling: observations imply an acceleration of the universe's expansion over the past few billion years. In order to explain such an acceleration, we need "dark energy" with large negative pressure to generate a repulsive gravitational force. The evidence comes from studies of the total energy density of the universe and from supernova observations. Precision measurements of the cosmic microwave background have shown that the total energy density of the universe is very near the critical density needed to make the universe flat (i.e. the curvature of space-time, defined in General Relativity, goes to zero on large scales). Since energy is equivalent to mass (Special Relativity: E = mc2), this is usually expressed in terms of a critical mass density needed to make the universe flat. Ordinary matter such as stars, dust, and gas account for only 5% of the necessary mass density. Observations have shown that dark matter cannot account for more than ~25% of the critical mass density. Both the microwave background and supernova observations suggest that dark energy should make up ~70% of the critical energy density. When added to the mass-energy of matter, the total energy density is consistent with what is needed to make the universe flat.