Final answer:
The universe is believed to be very close to flat and will likely expand forever, according to recent evidence which includes the cosmic microwave background radiation and expansion rate measurements. This aligns with the standard model of cosmology, grounded in general relativity, with the cosmological constant playing a key role.
Step-by-step explanation:
Understanding the Structure and Fate of the Universe
The question of whether the universe is open or closed deals with its ultimate fate and structure. For many years, physicists have debated whether the universe will continue to expand indefinitely, which is characteristic of an open universe, or eventually stop and perhaps collapse in a 'Big Crunch', which would be a feature of a closed universe. The introduction of the cosmological constant by Einstein, initially intended to prevent any expansion or contraction, later became a vital component in the modern understanding of the universe's expansion.
Recent experimental evidence suggests that the universe is very close to being flat. This is significant because a flat universe will continue to expand forever but at a rate that does not lead to closure. The cosmological constant, matter's curvature of space, and the inflationary scenario are critical components in this discussion.
Observations, such as measurements of the cosmic microwave background radiation (CMBR) and the expansion rate of the universe, have led most researchers to favor a model where the universe is barely open and negatively curved. This implies an unlimited universe where one could travel infinitely in any direction without returning to the starting point. In contrast, a positively curved, closed universe suggests that traveling far enough in any direction would bring one back to the starting point, akin to circumnavigating the Earth.
These considerations are grounded in Einstein's theory of general relativity and underpin the standard model of cosmology, which attempts to explain large-scale observations of the universe, including the structure of the CMB, abundance of light elements, and the apparent acceleration of the universe's expansion.