if the mass of everything in the universe doubles, can we detect this changes?

Yes, If the mass of the everything doubles than whole universe will get shrunk and the heavenly bodies in the space will collide and get destroyed. because more the mass more the gravitation force. chances gettin hitted by a asteroid will increase. People will find difficult to walk, aeroplane will crash down and satellites may fall down. we've to give the rockets more spped currentl which is 11.2 km/s. so many things happen.

If this happend within our solar system all life on earth would die very quickly, especially at the lower elevations via atmospheric pressure. More mass means more gravity, not to mention the effects this would have on our Sun.

Assuming this event happend outside our solar system, large planets would become stars, and large stars would become black holes. So yes we would observe a clear change.

Why not?? We can detect...but not with the pan balance and weight...instead you need to use the spring balance....Because in the pan balance...you put on the weight which is also doubled...but in the spring balance that is not the case....so using Spring balance you can detect the changes....Good Luck...try it...

Conservation of mass

the laws of thermodynamics

yes the inertia of every thing would increase making it harder to accelerate objects

We would all instantly get FAT!