# What evidence makes it obvious that the universe is flat? And why isn't there up and down in universe? ?

Why are we much confident in saying that the universe is flat? Also why is it accelerating constantly? As there is the role of density of space and matter... But don't know completely... Please explain in detail

### 9 Answers

- ไม่เป็นไรLv 71 month ago
According to the best measurements astronomers have ever been able to make, the curvature of the universe falls within a range of error bars that indicate it's flat. Future observations by some super Planck telescope could show a slight curvature, but for now, the best measurements out there say it flat like my wife.

There is an up and down in space. "Down" is simply the direction gravity is pulling you, and "up" is just the opposite direction. Since there is gravity everywhere in space, there is also an up and down everywhere in space.

- Jeffrey KLv 61 month ago
We don't see any curvature in the microwave background radiation. All triangles have 180 degrees. This doesn't mean that the universe is flat. It might have a curvature but be so big that we can't detect it. There is no way we can know.

- 1 month ago
The evidence is provided by the WMAP and Planck orbiters which measured the "curvature" of the universe by counting galaxies in slices of the universe, and found the universe to be flat within experimental error.

Put simply: the volume and hence galaxy content of successive slices of the universe will be different depending on whether the universe is curved or flat.

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- Anonymous1 month ago
These conclusions about cosmology are the result of analysis of thousands of astronomical observations of different types. In other words, it's not something that can be explained in a Y!A answer. You can learn if you really want to, but its about a year-long course at the graduate level, and you would need a bunch of physics as a prerequisite.

But the short answer is that the current best cosmological model is the result of fitting parameters to a manifold that satisfies the equations of General Relativity using essentially all existing astronomical data.

- nebLv 71 month ago
The universe can be modeled at cosmological scales using Einstein’s general relativity. In order to simplify the equations, certain assumptions are made to get exact solutions to VERY complicated equations.

The universe is assumed to be homogeneous, isotropic, and that it’s matter/energy density is usually represented as a perfect fluid in a comoving (with expansion/contraction) reference frame. The bottom line to that is that there is no momentum because frame is comoving, there is no shear, but there is a homogenous, isotropic energy density and PRESSURE. These are the terms that determine the curvature of the universe and whether or not it’s expansion is accelerating.

The average energy density of the universe determines its overall curvature - if the energy density is at the critical density, the universe has NO curvature. Greater than the critical density, the universe is positively curved, less than the critical density, the universe is negatively curved.

The pressure terms are a kind of negative pressure and help determine the expansion rate. The negative pressure is believed to be created by the effects of dark energy. If the pressure is greater than 3 times the energy density, the universe will accelerate its expansion (the 3 comes from the fact that pressure is isotropic so there are 3 components). Keep in mind that pressure is a source of gravity and that negative pressure gravitationally expands the universe.

It is believed that dark energy has a constant energy density so the total amount of dark energy has continuously increased as the universe has expanded. At some point about half way through the age of the universe, dark energy started to dominate over regular energy. At that point, the pressure exceeded the energy density causing the universe to accelerate its expansion. That is still going on today.

- MorningfoxLv 71 month ago
>> Please explain in detail

Sorry, but explaining in "detail" is going to take advanced college-level advanced math and physics. Plus many hundreds of hours of lectures and study. There just isn't room for all that here on Yahoo.

One of the main sources for evidence is from astronomical satellites, such as Planck. They measure the amount of mass in the observable universe, and there is just the right amount for the universe to be flat (within +/-0.4%).

Another method is to measure the angle subtended to a far object (such as a galaxy). We can figure the size of the object, the angle here on Earth, and the distance. These combine into a consistent measure of flatness:

tan(angle) = size / distance <== not true if universe is not flat.

- CarolOklaLv 71 month ago
When physicists and other scientists say the Universe, or rather the peacetime continuum is spatially "flat," they do NOT mean it is spatially or geometrically "flat" in only 2 spatial dimensions like a plane. They mean at cosmic scales of size the space time continuum is NOT mathematically curbed In 3 OR MORE SPATIAL dimensions. That INCLUDES height and depth. Atoms and atomic and subatomic particles have 3 spatial dimensions. At macro scales of size from microscopic to stars and planetary systems and galactic super clusters and galactic sheets?and voids the space time continuum can be "locally" curved because of mass, energy and gravity and the laws of relativity.

In science, particularly physics, it is VERY important to define your inertial frames of reference, what coordinate system or systems you are using and what the origin point of that coordinate?system is and the scale of that coordinate system. Coordinate systems and geometries are NOT constrained to being Euclidian and Cartesian coordinate systems. A coordinate system can be spherical and the geometry can be Riemann geometry or have a Lorentzian manifold topology. The Universe is NOT constrained to being Euclidean or Cartesian topology.

If this explanation is over your head and you do not understand the my vocabulary and is the equivalent of a foreign language, please let me know. I don't want to offend you by simplifying too much or insult you by being overeducated and too academic. I want to be able to communicate with you effectively.