# Speed of light in vacuum - is it really a constant and what is the accurate value? [duplicate]

Let's suppose that the speed of light would not be a constant but a function of something. As is quite clear now, universe is expanding exponentially. If the speed of light would actually be a function of this expansion, maybe in terms of cosmological constant... We would never find this out because the measure of distance (definition of meter) is now locked; meaning it is defined in terms of the speed of light! Also, in a way we know that the speed of light may indeed be a function of cosmological constant because very distant targets are escaping us faster than the speed of light? Is it hence indeed so that the speed of light just can't be measured, or defined currently by any independent means?

## marked as duplicate by honeste_vivere, ZeroTheHero, Jon Custer, Kyle Kanos, John Rennie cosmology StackExchange.ready(function() { if (StackExchange.options.isMobile) return; $('.dupe-hammer-message-hover:not(.hover-bound)').each(function() { var$hover = $(this).addClass('hover-bound'),$msg = $hover.siblings('.dupe-hammer-message');$hover.hover( function() { $hover.showInfoMessage('', { messageElement:$msg.clone().show(), transient: false, position: { my: 'bottom left', at: 'top center', offsetTop: -7 }, dismissable: false, relativeToBody: true }); }, function() { StackExchange.helpers.removeMessages(); } ); }); }); Aug 29 '17 at 7:12

The speed of light is now fixed due to the current definition of the metre and the second. Ir is now exactly 299,792,458$ms^{-1}$.