Assuming that Helium behaves as an ideal gas, that means that the Helium molecules have no intermolecular forces. They move freely without depending on one another's energy. We can use the ideal gas equation:
PV = nRT, where
P = pressure
V = volume
n=number of moles of gas
R = gas constant equal to 8.314 J/mol-K
T = absolute temperature
If pressure is kept constant, and we assume that the system is closed such that no moles are added or escaped, then the equation becomes
P/nR = T/V = k, where the k denotes constant. Therefore,
T = kV
So, when the volume doubles, the only reason for this is the increase also of temperature. This is because temperature is directly proportional to the volume of the gas. The answer is: The temperature increased.
