Who said they did? “A lexicographer studies words and compiles the results into a dictionary. This is one of several words for a certain type of writer or editor. Just as a playwright writes plays and a poet writes poems, a lexicographer puts together dictionaries.”
Vocabulary.com
When someone tells me that elephants are yellow reptiles living in Antarctica, I provide them this:
Elephant
A very large plant-eating mammal with a prehensile trunk, long curved ivory tusks, and large ears, native to Africa and southern Asia. It is the largest living land animal.
Oxford Dictionary
The lexicographer studies words and compiles the results so I don’t have to argue about what words mean.
When people say ‘the Big Bang theory is not a theory’ I first point them at a simple dictionary. That fixes the problem for most. Some keep making noises, so I have to give them a
better source. It's a shame they didn't pay attention to the dictionary, but, what can you do?
In the following definition of ‘Scientific Theory’, I have bolded the word ‘falsifiable’. If you then scroll down you’ll see that the Big Bang Theory is listed as an example of ‘Scientific Theory’ on the same page.
A scientific theory is an explanation of an aspect of the natural world that can be repeatedly tested and verified in accordance with scientific method, using accepted protocols of observation, measurement, and evaluation of results. Where possible, theories are tested under controlled conditions in an experiment. In circumstances not amenable to experimental testing, theories are evaluated through principles of abductive reasoning. Established scientific theories have withstood rigorous scrutiny and embody scientific knowledge.
The meaning of the term scientific theory (often contracted to theory for brevity) as used in the disciplines of science is significantly different from the common vernacular usage of theory.[4][Note 1] In everyday speech, theory can imply an explanation that represents an unsubstantiated and speculative guess,[4] whereas in science it describes an explanation that has been tested and widely accepted as valid. These different usages are comparable to the opposing usages of prediction in science versus common speech, where it denotes a mere hope.
The strength of a scientific theory is related to the diversity of phenomena it can explain and its simplicity. As additional scientific evidence is gathered, a scientific theory may be modified and ultimately rejected if it cannot be made to fit the new findings; in such circumstances, a more accurate theory is then required. That doesn’t mean that all theories can be fundamentally changed (for example, well established foundational scientific theories such as evolution, heliocentric theory, cell theory, theory of plate tectonics etc). In certain cases, the less-accurate unmodified scientific theory can still be treated as a theory if it is useful (due to its sheer simplicity) as an approximation under specific conditions. A case in point is Newton's laws of motion, which can serve as an approximation to special relativity at velocities that are small relative to the speed of light.
Scientific theories are testable and make falsifiable predictions. They describe the causes of a particular natural phenomenon and are used to explain and predict aspects of the physical universe or specific areas of inquiry (for example, electricity, chemistry, and astronomy). Scientists use theories to further scientific knowledge, as well as to facilitate advances in technology or medicine.
As with other forms of scientific knowledge, scientific theories are both deductive and inductive,[6] aiming for predictive and explanatory power.
https://en.wikipedia.org/wiki/Scientific_theory
Scrolling down the same page we see this:
Examples
Biology: cell theory, theory of evolution (modern evolutionary synthesis), germ theory, particulate inheritance theory, dual inheritance theory
Chemistry: collision theory, kinetic theory of gases, Lewis theory, molecular theory, molecular orbital theory, transition state theory, valence bond theory
Physics: atomic theory, Big Bang theory, Dynamo theory, perturbation theory, theory of relativity (successor to classical mechanics), quantum field theory
This shows you haven’t read the Big Bang Theory. See details about it
here.
Quote [emphasis mine]:
Features of the model
The Big Bang theory depends on two major assumptions: the universality of physical laws and the cosmological principle. The cosmological principle states that on large scales the universe is homogeneous and isotropic.
These ideas were initially taken as postulates, but today there are efforts to test each of them. For example, the first assumption has been tested by observations showing that largest possible deviation of the fine structure constant over much of the age of the universe is of order 10−5.[35] Also, general relativity has passed stringent tests on the scale of the Solar System and binary stars.
So is The Big Bang Theory a Scientific theory? You bet your ass it is.
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